Mental addition of two-digit numbers

I explain various basic principles we can use to mentally add two 2-digit numbers.

One of the main principles is that we can add in parts. For example, to add 60 + 23, we add 60 and 20 to get 80, and then add the 3 to get 83.

Then I compare single-digit additions to two-digit additions, and that gives us another strategy. For example, just like 8 + 6 goes over ten by four (14), even so, 28 + 6 goes over the next ten by four (and is 34).

The basic idea for adding two 2-digit numbers is to add them part-by-part. For example, to add 15 and 18, we add 10 + 10 = 20, then 5 + 8 = 13, and lastly add those two sums: 20 + 13 = 33.

But sometimes other "tricks" work. For example, to add 44 + 19, think of 19 as being 20. Then, 44 + 20 = 64, but that is one too much, so the real answer to our original problem is 63.

We also fill in a skip-counting pattern and solve some missing-number puzzles (which Mathy of course loves!)

This lesson is meant for 2nd or 3rd grade, or for anyone who hasn't learned these principles!

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See also

Two-digit mental addition — online practice

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

Number rainbow & fact families with 12

Math Mammoth Grade 2 curriculum

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