Mental subtraction strategies 1: Use known facts; subtract in parts
We can solve 23 − 8 and 93 − 8 (and 63 − 8 etc.) by our knowledge of the basic subtraction fact that 13 − 8 = 5. Think of it this way: in 13 − 8, the answer goes "down" to the previous ten, and ends in 5. It works the same way for 63 − 8, for example: the answer is in the previous ten (in the 50s) and ends in 5.
Besides using the basic subtraction facts, here is another strategy for subtracting a single-digit or two-digit number from a two-digit number: subtract in parts.
For example, to subtract 52 − 5, first subtract enough that you go "down" to the previous ten, or to 50. That means we subtract 52 − 2. Then we still need to subtract 3 more. So, we go 50 − 3 = 47.
In the video below, we look at several subtraction problems (2-digit and 3-digit) where the most effective strategy is to think of the difference between the numbers. This strategy works best when the numbers to be subtracted are close to each other.
The last example we look at is the two-digit subtraction 72 − 37. I solve it by finding the difference between 37 and 72 — essentially "adding up" from 37 to 72, in three different parts. I show arrows with jumps — like using a number line without tick marks.
See also
More mental subtraction strategies
Math Mammoth Grade 3 curriculum