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I explain two basic strategies for mental addition, using several examples. The first one is that of adding in parts. For example, to add 36 + 47, we can add 30 and 40, and 6 and 7. Since 6 + 7 = 13, this problem becomes 30 + 40 + 13, which is easier than the original.

Another strategy is to use a fact you already know to solve a similar addition problem. For example, if you already know that 6 + 8 = 14, you can use that to solve 76 + 8 or even 60 + 80.

How to quickly add 99 + 37? Compare it to the easy problem 100 + 37. The latter equals 137. The former is just one less, or 136. This strategy of using an easier problem is what we study in this lesson.

In the video below, I solve a PUZZLE CORNER with two unknowns, signified by a triangle and a square. We are given their sum and their difference. Yeah, you could use algebra but here in 3rd grade, we solve this puzzle using guess and check.