This lesson covers subtraction in three forms, in the visual, abstract and story form. You may choose to skip to the part that is best suited to your classroom needs or go through them one by one. Starting with the visual, discuss the steps of subtraction with your students. First you count the total number of objects, this is the first number in your subtraction problem (the minuend). Then you count how many are taken away, this is the second number of your subtraction problem (the subtrahend). When you take the second away from the first you are left with the difference. Demonstrate this with the ice cubes and the candles. Check that students are able to do this by asking them to subtract the cans and bananas. Then move to the abstract and discuss the different strategies they can use for subtraction. You could use a rekenrek or blocks, you can count back on a number line, or you could also see how many you need to add to the second number (subtrahend) to equal the first number (minuend). You can also subtract to an easier number (like ten) and subtract in two steps. Have students find the difference of the next subtraction problems. Ask them to explain their strategy. Finally walk students through the steps of solving a story problem. Make sure students know how to determine which numbers are needed for the math problem, what kind of math problem it is, to say or write the math problem, and finally to solve the math problem. Practice this as a class and then ask students to solve a story problem.
Check that students are able to subtract with numbers up to 20 by asking the following questions:
- What do you look at when you solve a story problem?
- How do you solve 13 - 4?