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.
If you cannot see the video above, click here for an alternative video player.
See also
Two-digit mental addition — online practice
Math Mammoth Grade 2 curriculum