Even though addition and subtraction are reciprocal operations, many students find subtraction to be more challenging than addition. Counting backwards creates opportunities for mistakes…and drawing the number and crossing off the amount to be subtracted is simply not efficient! Below you will find a few subtraction strategies explained along with a video. The goal is to use strategies that are efficient and accurate…and to use them only as long as you need them as support.
The Golden Arches strategy:
This strategy works best for subtraction of numbers between 12 and 20. It involves using 10 as an anchor number and taking two jumps to solve the problem. The ultimate goal is to internalize these amounts and no longer depend on documenting the steps.
Number Line strategy:
While similar to the arches strategy, using a number line allows for multiple jumps and works well for subtracting numbers with large differences between them. Larger differences might require more jumps at first, but taking lots of jumps is not the goal.
The importance of flexibility:
The strategies introduced above can be helpful when subtracting numbers, but there are many other strategies that work as well. It’s important to be flexible rather than relying on a rote routine to solve math problems. Understanding amounts and their relationships to known “anchors” sometimes makes number lines unnecessary. The video below shows three different approaches to a single problem. It’s important to think about the amounts you are working with and to have an idea about the approximate answer before you begin!
In all cases, it is important to be thinking about the amounts and the difference between them rather than relying on a single procedure. We want students to be able to do as much of this computation as they can in their heads, something that is possible when they understand the number amounts and can “see” them in different ways.
***A special thanks to our math/video helpers who made these videos to demonstrate the strategies.