Exercise 1 Page 5

Let's first think of examples of numbers that are easy to compute. 65+35&= 221-21& = 20 * 30 &= Notice that numbers than end in 5 are easy to add. Let's add them then! 6 5+3 5&=100 221-21& = 20 * 30 &= Numbers with the same digit or digits at the end are easy to subtract. This is because the result always ends in 0 s. 65+35&=100 2 21- 21& = 200 20 * 30 &= Also, multiplying numbers than end in 0 s is easier. To multiply a number by a multiple of 10, we ignore the zeros at the end and multiply the other digits first. Then, we just add the zeros to the product! 65+35&=100 221-21& =200 2 0 * 3 0 &= 6 0 0 See that all these these pairs of number go well together. If a couple of people go well together, we usually call them compatible.

The same thing applies to numbers! We call these pairs of number compatible, too. Let's complete the definition.