this post was submitted on 14 May 2024
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Multiplication of x times 6:
x * 6 = 1/2 x * 10 + x
This can sometimes be a shortcut for numbers that are easier to divide by 2 than to multiply by 6.
Take half as tens and add the number.
6 * 6 = 30 + 6 = 36
8 * 6 = 40 + 8 = 48
150 * 6 = 750 + 150 = 900
320 = 1600 + 320 = 1920
Etc.
Sleep well.
So an extension of the x * 5 = x/2 * 10 shortcut
Exactly. Multiples of 5 are easy enough in my opinion, but the principle can be used for all kinds of stuff when trying to calculate quickly.
For instance 9x =10x-x is usually faster than 9x (at least for my brain).
I once talked to an old guy who called it "little math", because it fits in your head instead of having to use paper and pencil at the desk. It must have been taught differently before I was born. I work with numbers, and I've often encountered these old geezers who can eyeball a number close enough to make a decision before I can boot my pc and put everything through Excel.
I like that there's a name for it. I always try to do that if possible. Division by 25? You mean multiply by 4 and divide by 100. Convert miles to km? That's x + x/2 + x/10.
Not sure if qualify as old geezer, you never know on the internet. I'm old for most people here, but you mention Excel, so you sound closer to my age :)