Left-to-Right Subtraction

Mental Math · Axiom Academy

LESSON Left-to-Right Subtraction Take the biggest place first, hold one running total — and let a place go negative when it needs to. 1. Biggest Place First, One Running Total Take 78 − 32 . Split both numbers by place and work down from the top. Tens: 70 − 30 = 40 . That 40 is your running total , and it already pins the answer to within 10. Ones: 8 − 2 = 6 , which lifts the total to 46 . One number, adjusted once per place — never two answers held side by side. A partial per place, folded into one total 2. A Place Is Allowed to Go Negative Now 84 − 37 . Tens: 80 − 30 = 50 , running total 50. Ones: 4 − 7 . On paper this is exactly where you would stop and borrow. Mentally, don't — just let the partial be −3 . A negative partial is a deficit : three that still has to come off. Fold it in and the total steps down from 50 to 47 . For 73 − 38 : tens give 70 − 30 = 40, and the ones give 3 − 8 = −5 . Fold in the deficit and 40 becomes 35 . The deficit got bigger; the method did not change. 3. Deficits Stack — and Each One Moves Less Three digits, same rule. Take 523 − 178 . Hundreds: 500 − 100 = 400 , and the size of the answer is settled. Tens: 20 − 70 = −50 , so the total drops to 350 . Ones: 3 − 8 = −5 , settling on 345 . Two deficits in a row, and neither one needed a borrow. Watch their sizes: the tens deficit moved the total ten times further than the ones deficit — every place you pass matters ten times less than the one before it. Borrow twice before you can start

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