Key Idea1 of 3
Using Doubles to Subtract
If you know 8 + 8 = 16, then 16 - 8 = 8! All doubles work this way. 7 + 7 = 14, so 14 - 7 = 7. 9 + 9 = 18, so 18 - 9 = 9.
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Math — Subtraction to 20
If you know 8 + 8 = 16, then 16 - 8 = 8! All doubles work this way. 7 + 7 = 14, so 14 - 7 = 7. 9 + 9 = 18, so 18 - 9 = 9.