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mrkickling
on Feb 8, 2014
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Show HN: Speedsums
I use to think of it like 10 and then subtract with the other factor. For example if you have 9x7, think 10x7 and then subtract 7. 70-7 = 63.
And if you have 9x9 just think 90-9=81.
9x10 = 100-10 = 90
9x15 = 150-15 = 135 and so on..
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And if you have 9x9 just think 90-9=81.
9x10 = 100-10 = 90
9x15 = 150-15 = 135 and so on..