First, students will learn a trick for mentally subtracting numbers within 100. Namely, if you have a problem like 84 - 59, you can add 1 to both numbers, giving 85 - 60. This problem will is equivalent to the original problem, and it's much easier to solve mentally. As another example, to find 82 - 34, we can subtract 4 from both numbers. Now we're thinking 78 - 30, which is again, easy to do mentally. We could have also taken 82 - 34 and added 6 to both numbers, which would have given us 88 - 40, which of course, gives us the same answer.

Next, students will practice mentally adding and subtracting within 100.

Conclude by leading this investigation:

Addition Boomerang 100 by MathPickle