Help, I'm having trouble multiplying numbers by 7 in my head

Discussion in 'Basic Math' started by janus, Dec 20, 2023.

1. janus

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Dec 20, 2023
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I don't know how to approach multiplying numbers by 7 purely inside my head without any external help, how do you multiply a number by 7 inside your head? say you're multiplying 5 by 7, do you turn the 7 into a number that's easier for you to multiply like 5, multiply 5 by 5 and then multiply 5 by the remaining 2 and then add the 25 and 10 together, i do that a lot but it doesn't work with bigger numbers for me? is there any other way to multiply by 7 purely inside your head?

janus, Dec 20, 2023
2. e.jane.aran

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Aug 13, 2023
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I hate the 7's, too. But at least some of the products involve other numbers that are easy. I mean, 7*1 is 7, right? And 7*2 is 14.

You've probably memorized the 9's, so you've got 7*9 = 63. The square is probably easy, too; 7*7 = 49. Also, 7*3 = 21 and 7*4 = 28 seem kinda easy, somehow. I know the 5's, so 7*5 = 35 is okay.

It's the 7*6 and 7*8 where I have trouble.

For 7*6, I cheat, I guess you could call it. I know that 6*6 = 36; 7*6 is one more 6 added on, so 7*6 = 36 + 6 = 42.

I haven't been able to find a trick for 7*8, other than knowing that there's a multiple of 7 that equals 56, so 7*8 must be the 56. But you could cheat for this one, too. You know that 8*8 = 64, and 7*8 is one 8 less than 64, so 7*8 = 8*8 - 8 = 64 - 8 = 56.

If you come across a better method, please return here and share!

e.jane.aran, Dec 21, 2023
janus and conway like this.