Start by discussing the importance of being able to subtract with three numbers, giving the practical example of needing to hand out candies to more than one person and wanting to know how many you have left. Next, explain to students how to solve a subtraction problem with multiple numbers (subtrahends) using blocks. Tell the students to first look at how many there are in total. This is the first number (minuend) of the subtraction problem. Then take the next number (subtrahend) and subtract that from the first number. Use that difference and subtract the final number (subtrahend) from the intermediate difference. Discuss with students that the order of the second and third numbers (subtrahends) does not matter, as long as the first number(minuend) stays in the same place at the start of the subtraction problem. Different methods to solve these subtraction problems follow on the interactive whiteboard like using the rekenrek or blocks. Discuss these as a class.
Check that your students understand subtraction to 10 with three numbers by asking the following questions:
- Why is it useful to be able to subtract multiple numbers from an original starting number?
- How do you subtract a subtraction problem with three numbers?
- Why can it be useful to swap the order of the second and third numbers (subtrahends) in a subtraction problem?