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QC (in Berlin 7/29-???)

i have the answer memorized but two equivalent approaches: 1) factor 18 as 9x2, so 18x5 = 9x2x5 = 9x10 = 90 2) multiplying by 5 is equivalent to dividing by 2 and multiplying by 10. so you get 9 and then 90 these are math competition mental math tricks, i got more

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@ABlondeAvocado

· Oct 21, 2021

Math teacher with a question: Using only mental math, solve 18x5 What steps did you take in your head to get to the answer?

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5:36 PM · Oct 22, 2021Twitter for iPad

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(20-2)x5 = 100 - 10

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@prerationalist

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i was thinking 20 fives is a hundred, so 18 would be ninety

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QC (in Berlin 7/29-???)

that’s a good one too! also 18=10+8

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whenever there’s a 5 on one side and an even number on the other side, hoho it’s on, baby

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even if it’s like say 18*35, by moving the 2 over we get 9*70, 9*7 is 63, so 630

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Oh I really like (2)…I did 18=10+8, 10*5=50, 8*5=40, 50+40=90 but it’s just more steps than your (2).

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These are neat!

@BadgerDreaming

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9*5*2. 9*2*5 would just give me 18*5 again and feel silly

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@BadgerDreaming

Besides generally if I factor something down to simplify I gotta get those parts away from each other so they don't team back up

@PlaneHarvester

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Thank you, this is so cool!