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Problems or situations in daily life can often be represented by algebraic expressions. When an expression is set equal to another expression or number, it is called as equation. This lesson will be an introduction to equations and how to use them to model real-life situations.

### Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

Challenge

## Finding the Correct Floor

Tadeo and his little brother Dylan decide to go out together. They get in the elevator and Dylan randomly presses buttons on the panel until Tadeo stops him.

External credits: Icongeek26

The elevator went floors up and then floors down before arriving at the ground floor, or Floor Which floor were Tadeo and his brother on when they got in the elevator?

Discussion

## Equation

An equation is a type of mathematical relation that indicates that two quantities are equal. Equations often contain one or more unknowns values called variables. Some examples are shown below.
Solving an equation with a variable means to find the value or values of each variable that make the equation true.
Discussion

## Solution of an Equation

A solution of an equation is a value that makes the equation true. It satisfies the equation when it is substituted for the variable of the equation. Consider the following equation.
The equation has the solution because is the only value of  that makes the equation true. This means that the the right and left-hand sides of the equation are equal to each other when this solution is substituted for the variable.
For the other values that are not solutions, the right-hand side of the solution is not equal to the left-hand side. This is shown by a not equal sign Consider the result of substituting into the equation.
Some equations are impossible to solve. Consider the following example.
Notice that it is impossible to get a number itself after adding to that number. This means that this equation has no solution. Conversely, it is possible for an equation to have more than one solution. An equation can also have infinitely many solutions, which means that all values satisfy the equation.
This equation will be true for all values of since the expressions on the left-hand side and right-hand side are equivalent expressions.

### Extra

Replacement Set
Sometimes the solution set of an equation can be obtained from a replacement set, which is a list of possible solutions. The values from the replacement set are substituted into the equation and then checked to confirm whether they satisfy the equation or not. Consider the following equation and its replacement set.
In the following table, each value is substituted into the equation and the equation is evaluated.
Value of  Substitute Are both sides equal?

As shown in the table, the only solution of the equation is

Example

## Deciding What to Order

Tadeo and his brother Dylan are very hungry and decide to go to one of the restaurants at the city center. There are four different kinds of combos on the menu.

a Suppose they order the same combo. The price of their combo satisfies the equation Which combo did they choose?
b Now suppose they order different combos. Both combo prices satisfy the equation Select the combos they choose.

### Hint

a Substitute the given combo prices into the equation.
b Substitute the given combo prices into the equation.

### Solution

a Consider the given equation again.
There are four different price points on the menu. These are and If Tadeo and his brother order the same combo, just one of these numbers will satisfy the given equation. With this in mind, substitute each of the prices into the equation, one at a time, and simplify. Start with
Since makes the equation true, it is a solution to the equation. Now, check the remaining prices by substituting them into the equation and simplifying to see whether the equation has more solutions.
Combo I
Combo II
Combo III
Combo IV

As shown in the table, the other prices do not satisfy the equation. This means that Tadeo and his brother both ordered Combo I.

b Since the brothers ordered different combos this time, there is a different equation to consider. Even though the equation might look intimidating, the solution process is the same — substitute the values into the equation to see if they produce a true statement. Two or more combo prices will satisfy this given equation.
Since does not satisfy the given equation, it is not a solution. This means that neither boy ordered Combo I. Check the remaining combo prices by following the same method.
Combo I
Combo II
Combo III
Combo IV

According to the table, the equation has two solutions, and This means that one of the brothers ordered Combo II and the other one ordered Combo IV. Bon appetit!

Pop Quiz

## Checking Solutions

Consider the given equation and value for the variable. Determine whether the value is a solution to the equation.

Example

## Writing Equations

After having lunch, Tadeo and his brother Dylan walk around the city center. They start to discuss their physical characteristics.

a If Tadeo is centimeters tall, write an equation to find the height of his brother.
b If Tadeo is kilograms, write an equation to find the weight of his brother.
c If Tadeo is years old, write an equation to find his brother's age.

### Hint

a Tadeo is is centimeters taller than Dylan is. This involves adding some numbers.
b Three-fourths of a number is the same as the product of and that number.
c Difference means subtraction.

### Solution

a Consider the conversation between Tadeo and his brother. Tadeo says that he is centimeters taller than Dylan is. This means that Tadeo's height is centimeters more than his brother's height. In order to write an equation that represents this situation, carefully examine the sentence again.
The word is represents an equals sign, which means that the expressions to the left and right sides of this word are equal to each other. Tadeo is centimeters tall, so write this number on the left-hand side of the equation in place of

Next, more than indicates the addition of numbers. Let be the variable that represents Dylan's height to write an expression for the right-hand side of the equation.

There may be other ways to write this equation. Saying that Dylan's height is centimeters less than Tadeo's height has the same meaning. Here, less than refers to subtraction. Consider how to rewrite the equation as a subtraction expression.
Both of the equations can be used to find Dylan's height.
b Once again, start by considering Dylan's claim. Note that three-fourths of a number means the product of and that number. With this in mind, write the sentence as an equation. Let be Dylan's weight.
Dylan's weight can be found by using the translated equation.
Note that there are other possible ways to write the fraction
This means that the equation also represents the given sentence.
c This time, an equation that represents the sentence, "The difference between Tadeo's and Dylan's ages is " must be found. Tadeo says that he is older than Dylan, and Tadeo is years old. The word difference means subtraction. Let be the Dylan's age.
This situation can also be represented by other equations. If Tadeo is years older than Dylan, we know that his age is Dylan's age plus This means that the equation can be written using addition as well.
Both equations can be used to find Dylan's age. Also, keep in mind that there is not only one way to represent a situation by an equation. Equations can be rewritten as several different equations by rearranging the terms of the equation.
Example

Dylan and Tadeo like playing collectible card games and video games. They stop at a game store and see what is new.

a Dylan decides to purchase individual cards. After this purchase, he has cards. How many cards did Dylan have before he bought the new ones?
b While Dylan is passionate about collectible card games, Tadeo prefers video games. After buying three new games for Tadeo is left with in his wallet. How much did he have in his wallet before he bought the games?

### Hint

a The number of cards Dylan has before before going to the store is less than

<listcircle icon="b">Tadeo started with more than the $\32$ he has left after buying the games.

### Solution

a Let represent the number of cards that Dylan has before buying more. After buying cards at the shop, he now has cards. This means that the number of cards he had before going to the store is less than This information can be written as an equation.
Now simplify the equation to find how many cards he had.
This means that Dylan had collectible cards before buying more at the mall. This also means that is the solution to the equation.
b Tadeo has a certain amount of money in his wallet before going to the store. After he spends he is left with This means that the initial amount is more than the money left after buying the new games.
Let be the initial money. Substitute the given amounts into this model to write an equation.
Now simplify the equation.
Example

## Selling Old Toys

Dylan and Tadeo have lots of old toy cars from their childhood. They decide to sell their old cars in a secondhand shop. They hope that other children will play with their old cars!

a The store gives them for cars. What is the cost of a single car if the shop pays the same amount for each car?
b How many cars would the boys need to sell in order to earn in total?

### Hint

a The boys received times the amount of money paid per car to get
b Use the cost of a single car that was found in Part A. Write a division equation to represent the situation.

### Solution

a The boys sold toy cars to the secondhand shop and received
The store paid the boys the same amount of money for each toy car. Since the boys received times the amount of money paid per car to get the amount of money the boys were paid per car can be found by dividing by
Now simplify the equation to find how much the boys were paid for each toy car they sold.
Since represents the amount of money the boys receive per toy car, this means that the shop paid the boys for each toy car. This number is also the solution to the equation.
b This time the boys want to know how many cars they would need to sell in order to earn Remember that they will be paid for each car. Draw a diagram to represent this situation.
Consider how to split in groups of This will require division — in other words, divide the total amount that will be earned by the price of a single car to find the number of cars sold. Let be the number of cars.
Now calculate the quotient to find the value of
This means that they need to sell toy cars to get Keep in mind that the scenarios from Part A and Part B can be solved by using other methods. These solutions are just examples of one way of how to find the required information.
Closure

## Finding the Correct Floor

At the beginning of the lesson, Tadeo and his brother got into an elevator to go out together. Dylan randomly pressed the elevator buttons until Tadeo stopped him.

External credits: Icongeek26
The elevator went floors up and floors down before arriving at the ground floor, or Floor Which floor were Tadeo and his brother on when they got in the elevator?

### Hint

The ground floor is represented as Floor The boys went floors down before getting off the elevator.

### Solution

After the boys got in the elevator, it first went up floors, then down floors. Start by drawing a diagram to represent this situation.

Keep in mind that the ground floor is represented as Floor Since the boys went floors down before getting off the elevator, they must be at the floor after going floors up.

Recall that the boys reached the floor after going floors up. This means that they must be at a floor that floors down from the floor when they get in the elevator. Write this situation as an equation.
Now simplify this equation to find the floor the boys were on when they got in the elevator.
This means that the boys got into the elevator on Floor Notice that this is also the solution to the equation.