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Interesting.. this is how I do subtraction in my head. Didn't realize it wasn't the "standard" approach :).
by jm__87 9y ago
Interesting.. this is how I do subtraction in my head. Didn't realize it wasn't the "standard" approach :).
- srtjstjsj 9y agoThis is how it's taught at my local school, but it's a bit hidden, because the the 7 is "broken apart", not "subtracted": 61 - 17 = 60 + 1 - (10 + 1 + 6) = (60 - 10) + (-6) "Breaking apart" is equivalent to subtraction, but feels different when doing mental calculation.
- Zecc 9y agoAs do I, sort of. To subtract mentally I adjust the numbers of both sides by the same amount so it rounds up nicely in either side. In this case I'd go: 61-17 =>(-1 on both sides)=> 60-16 =>(-10 on both sides)=> 50-6 =>("normal" borrow)=> 44 Had it been, say 148-34, I could go either: 148-34 =>(+2)=> 150-36 =>(-6 with borrow)=> 144-30 =>(-30) 114 or most likely 148-34 =>(+2)=> 150-36 =>(-30)=> 120-6 =>(-6) 114 though in this simple case I'd just 148-34 =>(set the 100 aside)=> 48-34 =>(split digits)=> 40-30 + 8-4 = 10+4 =>(join digits again)=> 14 =>(put the 100 back in)=> 114 This is in line with answers that were given to the Math Exchange question. Edit: @srtjstjsj put it nicely when they call it "breaking apart".