How to use mental strategies to subtract within 20 using the "break down to 10" strategy? With practice, this strategy can help students gain fluency when subtracting within 20. By now children should be very comfortable with the sum-equals-ten addition and subtraction facts, such as 4+6=10 and 10-7=3.
Write 15-8=? on the board. Explain that:
1. 15=10+5;
2. 8=5+3;
3. 15-8=10+5-5-3=10-3=7.
Answer: 15-8=7.
