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!
If you cannot see the video above, click here for an alternative video player.
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