3.3.10 Using a Related Problem

4. Solving an Easier, Related Problem and then Compensating

You can solve a problem where you already know the answer and adjust. For example, when solving , you may know off the top of your head that , and then you might reason that subtracting 13 from 169 is 156, the closest multiple of 13 to 169, without going over 159. In other words, , which is three less than 159, so . Activity: Paper Supplies

A college bookstore buys pads of legal paper in bulk to sell to students in the law program at a cheap rate.

Each pack of paper contains 20 pads. If the store wants 1500 pads for the term, how many packs should be ordered? Discussion

We need to find how many groups of 20 are in 1500, so the calculation is:

If you imagine dividing some quantity of objects into 20 piles, one way to do it would be to divide it into ten piles instead, and then divide each of those piles in half. So, dividing a number by 20 is the same as dividing by ten, and then dividing the results by two. 