That's meant to be "12 + 3 = 15" (or maybe it's an intentional error to make the thing look more confusing?). The idea behind this particular subtraction algor

That's meant to be "12 + 3 = 15" (or maybe it's an intentional error to make the thing look more confusing?). The idea behind this particular subtraction algorithm is to reinforce two number-sense ideas: 1) a subtraction problem like "32 − 12 = ?" is equivalent to an addition problem "12 + ? = 32", and 2) we can group additions in ways that are convenient to use numbers that are easy to add or to use convenient intermediate results that are multiples of 5 or 10 or 100. EDIT: to fix my own math typo. :P