# Build on what they know

by Judy Storeygard

Many years ago, a friend of mine was studying for a test that her employer required for a promotion. Unfortunately, she had a real block about math, especially subtraction. She had asked if my son, at the time a high school student who liked math, would help her. My first thought was, “Oh no, but it comes easily to him. How will he be able to help her?”

Luckily, my fear was unfounded. Because he was curious about math, he watched how she was approaching the problems. He saw that her instinct was to add up instead of subtract, mostly because the borrowing algorithm did not make sense to her. However, she did not have confidence in her method, and didn’t fully understand why adding up could work. My son showed her how adding up was indeed a reliable method. He drew on her number knowledge to help her see that she could use easier-to-add landmark numbers as she added up. For example, to solve 56 – 37, she might first add a number like 20 (easy because it is a multiple of 10) to the 37, and then subtract 1 to arrive at the correct answer of 19. Or, she could have also added up in steps to get the same result, like this:

1. Add 3 to the 37 to reach the landmark number 40.
2. Add 10 more to get up to the landmark number 50.