Quick Subtraction Summary

Mental Math · Axiom Academy

SUMMARY Quick Subtraction Summary A look back at the eight routes through this unit — and how the numbers themselves tell you which one to take. Every subtraction is an addition question in disguise: 83 - 47 really asks what you add to 47 to reach 83 — and for most people that is the easier question. There is no single best method. The numbers tell you which to reach for: a small gap or a row of zeros on top invites counting up; a subtrahend just under a round ten invites same change or compensation. Every route here aims at the same target — a round number. Counting up hops to them, same change slides onto them, breaking apart bites down to them. Compensation's direction is the opposite of addition's. Round the subtrahend up and you took away too much, so add it back: 73 - 30 + 1 = 44 . Same change hands nothing back. Slide both numbers the same way and the difference never budges: 72 - 38 = 74 - 40 . Working from the left pins the answer's size before you touch a small digit, and a place is allowed to go negative — fold the deficit in rather than borrowing. Zero is a place you pass through, not a floor. Subtract the smaller from the bigger, then attach the minus sign: 23 - 58 = -35 . A plus sign in front of a negative is only an instruction to move left, so 8 + (-5) = 8 - 5 . Same signs pile up and keep that sign; different signs take the difference and keep the bigger side's. Core Concept Thinking Addition

This is the written version of the interactive lesson above. See the full Mental Math course.