Mental addition — add a 2-digit and a 1-digit number (with regrouping)

In this video lesson, I teach several strategies for adding mentally a two-digit and a single-digit number. For example, to add 38 + 7, we can add 8 + 7 = 15, and lastly add 30 and 15.

Another way to think about the same is that since the sum of 8 + 7 goes five OVER the next ten, then the sum of 38 + 7 also goes five over the following ten — which is 40 in this case — so the answer is 45.

The "nine-trick" works, also. For example, to add 29 + 6, we can think of 29 wanting to become 30, so it "takes" one from 6, which then becomes just 5. The addition is now changed to 30 + 5, which is easy to solve; it is 35.

These mental math techniques are taught in 2nd grade in the Math Mammoth curriculum.

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See also

Two-digit mental addition — online practice

Math Mammoth Grade 2 curriculum

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