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| | 12 Theory slides |
| | 13 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
Zosia draws a rectangle to represent the product of two whole numbers. The rectangle has a width of 2 units and a length of 3 units. Additionally, the area of the rectangle is the same as the product of the two numbers, which is 6.
What would the product be if the side length of each square was 0.1 unit long?In that case, could the rectangle be used to represent 0.2* 0.3? Help Zosia answer the following questions.
A place value chart is a table that displays the correct place of a digit in a number. Place value charts are useful for ensuring that digits are aligned correctly. For example, consider a decimal number. Decimal Number 673.452 The digits in this decimal number can be written in the place value chart to check their position. The aim here is to identify the positional values of different digits in the number accurately. It can be easier to start by writing the digits around the decimal point.
The rules for adding and subtracting decimals are similar to the general rules for adding and subtracting integers. The most important point is to align the decimal points correctly. For example, consider adding the following decimal numbers. 3.4 + 12.802 There are three steps to finding this sum.
The numbers are now written vertically. A place value chart can also be used to align the decimal places.
Now that the decimal digits in each of the numbers are equal, it will be easier to add or subtract these numbers.
The sum of the given numbers is 16.202.
Zosia likes singing. She decides to record a video of herself singing.
She records two video clips. The first video clip is 4.56 minutes long and the second is 5.54 minutes long.
& 4.56 + & 5.54 There is no need to add zeros to the end of either number here because they both have the same number of decimal places. Ignore the decimal point for the moment and continue adding as usual. Then, finally, add the decimal point to the sum.
The sum of the decimals is 10.10, or just 10.1. This means that Zosia's video is 10.1 minutes long.
& 10 .1 - & 1 .27 Notice that the numbers do not have the same number of decimal places. To make the subtraction easier, add one zero to the end of the first number so that both numbers have an equal number of decimal places. & 10 .1 0 - & 1 .27 Now ignore the decimal points and continue subtracting as usual, then place the decimal point in the difference.
The difference of the decimals is 8.83. This means that the final length of the video is 8.83 minutes.
Recall the steps to follow to add and subtract decimals. First, align the decimal numbers by their place values, one below the other. Then, perform the operation just like as if the numbers were integers. Finally, place the decimal point to the answer. Follow these steps to find the sum or difference shown in the applet.
Estimation is used to find a value that is close to the exact value of a product. This can be done by rounding the factors in the product to the greatest place value — the first non-zero digit on the left in a number. For example, consider the product of two decimal numbers. 35.92 * 0.014 Estimating this product can be done in two steps.
Next, make all digits to the right of the greatest place value zero. For 35.92, the digit to the right of the greatest place value is 5. For 0.014, the digit to the right of the greatest place value is 4.
| Estimate 35. 92 * 0.014 | ||
|---|---|---|
| Factor | Greatest Place Value | Digit to the Right |
| 3 5.92 | Tens | 5 |
| 0.01 4 | Hundredths | 4 |
The first factor will be rounded to the nearest ten. Since 5 is greater than or equal to 5, add 1 to the number in the rounded place 3. All place values to the right of 3 are replaced with zeros. 35.92 ≈ 40.00 (or 40) Similarly, for 0.014, the digit to the right of the greatest place value is 4. This is less than 5, so 1 remains the same and all place values to the right of 1 have a value of zero. 0.014 ≈ 0.010 (or 0.01) The numbers have now been rounded to their greatest place values. The procedure is visualized in the following applet.
Multiplying decimals has the same procedure as multiplying integers. The only difference is where the decimal point is placed in the product. The procedure will be demonstrated with an example. 1.69 * 3.7 The following steps can be followed to multiply decimals.
The sum of the decimal places is 3.
The product of 1.69 and 3.7 is 6.253.
Zosia shares her video on a video sharing platform.
Mark, Zosia's brother, promises to pay Zosia $1.75 for each view of her video. So far, 59 people have watched the video.
59 * 1.75 Each factor will be rounded to the greatest place value to estimate this product. In the number 59, the digit with the greatest place value is 5. In the other factor, the digit with the greatest place value is 1. 59 * 1.75 Now, take a look at the digit to the right of the greatest place value.
After that, make all digits to the right of the greatest place value zero. Starting with the first factor, the digit to the right of the underlined number is 9. This is greater than 5, so the digit in the rounded place is increased by 1 and the other digit is changed to a 0. ccc 5 9 & * &1.75 ↓ & & 60 && For 1.75, the number in the tenths place is 7. Since 7 is greater than 5, the number in the rounded place 1 is increased by 1 and the other digits are converted to 0. In this case, the zeros can be ignored because they will not change the value of the product. ccc 59 & * &1. 75 ↓ & & ↓ 60 & * &2 The product of 60 and 2 is 120. Mark will pay about 120 dollars.
59 * 1.75 The steps for multiplying these numbers is the same as multiplying two decimal numbers.
Start by multiplying 59 and 175.
The product of the numbers is 10 325. Since there are two decimal places in 1.75, the product of 59 and 1.75 must be a decimal number with two decimal places. r 59 * 1. 75 103. 25 l→ → ← r 0 decimal places + 2 decimal places 2 decimal places The product is 103.25. This means that Mark will pay Zosia 103.25 dollars.
In addition to the amount that Mark gives her, Zosia also earns money per view from the video sharing platform.
The platform pays 0.113 times what Mark paid for 59 views.
103.25 * 0.113 The digit to the right of the greatest place value determines how the number is rounded.
After rounding, remove any decimal digits and convert the non-decimal digits to the right of the greatest place value into 0. Since the digit with the greatest place value is the leftmost non-zero digit int he number, 103.25 has its greatest place value in the hundreds place and 0.113 in the tenths place. 103.25 * 0.113 In 103.25, the digit in the tens place is 0. Since 0 is less than 5, the digit in the hundreds place 1 remains the same and the other digits are replaced with 0. This results in the estimated number 100. ccc 1 03.25 & *& 0.113 ↓ && 100 && In 0.113, the number in the hundredths place is 1. Since 1 is less than 5, the number in the tenth place 1 is kept unchanged. Then, the other digits are replaced with 0. Since these zeros do not change the value of the number, they can be omitted. ccc 103.25 & *& 0.1 13 ↓ &&↓ 100 & * & 0.1 Multiplying a number by 0.1 means dividing the number by 10 because 0.1 = 110. This means that the product of 100 and 0.1 is 10.
The platform pays about $10 for 59 views.
Start by multiplying 10325 and 113.
There are 2 decimal places in the first factor 103.25, and there are 3 decimal places 0.113. This means that the product of the decimal numbers must be a decimal number with 2+3=5 decimal places.
The product of the numbers is 11.66725. Since money is represented by numbers with two decimal places, 11.66275 is rounded to two decimal places. 11.66275 & Round ⟶ & 11.67 The platform pays 11.67 dollars for 59 views. This is close the estimate found in Part A, so this answer is reasonable.
Multiply the decimal numbers. Make sure that the decimal point is placed correctly!
Mathematical operations such as addition, subtraction, and multiplication are performed on decimal numbers in a similar way that they are performed on integers. The most important point here is where to place the decimal point. Now consider Zosia's rectangle that can be used to represent the product of two numbers.
Since the area of a square is the product of its two sides, this square has an area of 0.1* 0.1 square units. 0.1 * 0.1 Now multiply the decimals as if they were whole numbers. 1 * 1 =1 Then, count the number of decimal places in each factor and add them. rr 0. 1 * 0. 1 & l→ → & r 1 decimal place + 1 decimal place 2 decimal places The number of decimal places in the product must be 2. Insert zeros to the left of 1 and move the decimal point two places to the left. rr 0. 1 * 0. 1 0. 01 & l→ → ← & r 1 decimal place + 1 decimal place 2 decimal places The area of a single square is 0.01 square unit.
In the previous part, it is found that the area of a each square is 0.01.
Since there are 6 small squares, the area of the rectangle can be found by multiplying 0.01 by 6. Multiply the numbers as if they were whole numbers. 1 * 6 = 6 The sum of the number of decimal places in each factor is 2. This means that the number of decimal places in the product must be 2. rr 0. 01 * 6 0. 06 & l→ → ← & rr & 2 decimal places + & 0 decimal places & 2 decimal places The area of the rectangle is 0.06. This is also equal to the product of 0.2 and 0.3. Alternatively, the Commutative and Associative Properties of Multiplication can be used to find the product.
Rewrite 0.2 as 2 * 0.1
Rewrite 0.3 as 3 * 0.1
Associative Property of Multiplication
Commutative Property of Multiplication
Associative Property of Multiplication
Multiply
Multiply
Either way, the answer is the same!
Paulina finds the difference of 8.7 and 6.27 as shown in the diagram.
Is Paulina correct?
We are given a diagram that shows how Paulina subtracts two decimal numbers.
We want to determine whether she correct. We can see that Paulina correctly lined up the decimal points. However, she did not add a zero to the first number. Additionally, she brought down 7 in the hundredths place of the second decimal number like it was an addition. Let's now complete the steps that Paulina skipped and find the difference.
We got a different result. This means that Paulina is not correct and the correct answer is 2.43, which corresponds to option B.
Tiffaniqua's family has a triangular garden. The dimensions of the graden are shown in the diagram.
Tiffaniqua's family wants to fence off their garden. What is the length of the fence needed to enclose the garden?
We need to add up the lengths of each side of the garden to find the length of the fence that they need to enclose it. 19.4 + 25.63 + 26.75 Let's first add the first two terms. We line up the digits according to their place values. Note that we need to add a zero to the first number so that each place has a digit. & 1 9 . 4 0 + & 2 5 . 6 3 We will add the decimals just like we do with whole numbers and place the decimal point in the answer.
Next, we will add the third side to the sum of the first two terms. 45.03+ 26.75 Again we will line up the decimal points so that place-value positions line up with each other. In this case, both numbers have the same number of decimal places, so we do not need add any zeros. & 4 5 . 0 3 + & 2 6 . 7 5 We will now add the decimals just like we do with whole numbers and place the decimal point in the answer.
This means that the family needs 71.78 meters of fencing to enclose the garden.
Jordan finds the result of an addition expression.
Jordan thinks that she can rewrite it as a multiplication problem. 3.14 * = 12.56 What should the missing factor in the equation be?
Jordan found the result of the given addition expression. 3.14 + 3.14 + 3.14 + 3.14 = 12.56 We want to find the missing factor of the product equal to the sum. 3.14 * = 12.56 We can see that the sum of the same four addends is equal to 12.56. Since the addends are the same and equal to each other, we can write the addition as a multiplication of four times 3.14. 3.14* 4 = 12.56 This means that the number we are looking for is 4.
Estimate the product of the numbers in each group by rounding them to the greatest place value. Which options have an estimated product of 60? Select all that apply.
Let's estimate the product of the numbers in the first group. We can see that for all factors, the greatest place value is the ones place. 5. 9 & * & 4. 9 & * & 6. 9 For each of the factors, the digit to the right of the greatest place is 9, which is greater than 5. This means that we increase the digit in the ones place of each number by 1 and ignore the decimal parts. 5.9 & * & 4.9 & * & 6.9 ↓ & & ↓ & & ↓ 6 & * & 5 &* & 7 The product of these rounded numbers is 210, so the product of the original numbers is about 210. We can eliminate option I since it does not equal approximately 60. Let's round the numbers in the other groups and estimate their products.
| Group | Round Numbers | Multiply |
|---|---|---|
| I | ccc 5.9 & → & 6 4.9 & → & 5 6.9 & → & 7 | 6 * 5 * 7 = 210 |
| II | ccc 2.9 & → & 3 4.1 & → & 4 5.1 & → & 5 | 3 * 4 * 5 = 60 |
| III | ccc 2.1 & → & 2 4.9 & → & 5 5.9 & → & 6 | 2 * 5 * 6 = 60 |
| IV | ccc 1.9 & → & 2 5.9 & → & 6 9.4 & → & 9 | 2 * 6 * 9 = 108 |
| V | ccc 1.15 & → & 1 12.3 & → & 10 5.95 & → & 6 | 1 * 10 * 6 = 60 |
The numbers in the groups II, III, and V have a product of about 60.
The weight of an object on Mars is about 0.38 of its weight on Earth.
Perseverance, a 1.025-ton Mars rover, landed in Jezero crater on February 18, 2021. How much does Perseverance weigh on Mars?
We want to find the weight of the Perseverance rover on Mars. We can do this by multiplying 1.025 by 0.38. 1.025* 0.38 We will multiply these numbers just as we do with two whole numbers to find the product. Let's ignore the decimal points for the moment. lr & 1 0 2 5 * & 3 8 & 8 2 0 0 + &3 0 7 5 0 & 3 8 9 5 0 Now we can place the decimal point in the product by counting the number of decimal places in each factor. The number of decimal places in the product is the sum of the decimal places in the factors. cr & 1. 025 * & 0. 38 & 8200 + & 30750 & 0. 38950 l ⟶ ⟶ ⟵ cl & 3 decimal places + & 2 decimal places & 5 decimal places This means that a 1.025-ton vehicle weighs about 0.3895 tons on Mars.