Thinking Addition for Subtraction

Mental Math · Axiom Academy

Thinking Addition for Subtraction Every subtraction is an addition question wearing a disguise — and the addition is the one you already know by heart. Subtraction is a question about addition Ask someone for 15 - 7 and you can often watch them count backwards, one careful step at a time. Ask the same person what 7 + 8 is and the answer comes out instantly. That is a little strange, because those are the same fact. Subtraction feels slow largely because of the way the question is worded — and you are allowed to reword it. Watch the question rearrange itself. The three numbers never change and nothing is added or removed; they simply trade places across the equals sign, and the minus is replaced by a plus. What starts as "take 7 away from 15" ends as "what do I add to 7 to reach 15" — and that second question you can already answer. Same three numbers, same fact — asked from the other end. Set a whole, then set how much is taken away. The subtraction on top and the addition underneath sit in the very same five slots, so you can see the swap at a glance. Whichever numbers you choose, the blank holds the same value in both lines — because they are one fact, not two. Try 20 - 13 or 18 - 9 and read the bottom line first — for most people the addition is the easier question. Three numbers, one fact, every question

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