Two-Digit Subtraction Examples

Mental Math · Axiom Academy

EXAMPLE Subtracting 73 - 28 by Counting Up Turn the subtraction into a short forward walk: hop from 28 up to 73, then add up the hops. Work out 73 - 28 in your head with the counting up method from the last lesson: stand on 28 , hop forward to friendly numbers until you reach 73 , and add the hops. Nice work — you found a two-digit difference without taking anything away or borrowing once. Remember: A difference is a distance: 73 - 28 is the gap from 28 up to 73 , so you can walk it forwards instead. Hop to friendly stops: first hop reaches the next round ten (that's a complement from Unit 1), middle hops run ten to ten, and the last hop is the target's ones digit. Add the hops: the running tally IS the difference — 2 + 40 + 3 = 45 . You get a free check: read it forwards and 28 + 45 = 73 . The route is yours to pick: 91 - 47 walks +3 then +41 , or +40 then +4 — both total 44 , because the endpoints never move. Counting up is the first of several subtraction moves in this unit — compensation, left-to-right subtraction, the same-change trick and breaking apart the subtrahend are all still to come.

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