# The Mathematical Ninja's Guide To Roundish Numbers

“So that’s 0.9332 times 500,” said the student. Dubiously: “In your head?”

“466.6,” said the Mathematical Ninja, before the student could reach for his calculator.

“Whoa,” said the student, some seconds later, once he’d tapped in the sum and found the S<=>D button.

The student was new here. He was still impressed by this stuff. The Mathematical Ninja tried very hard not to treat him like a simpleton. “Five hundred is half of a thousand,” he said, through gritted teeth ((he wasn’t cross; he was just experimenting with ways to avoid frostbite)). “So, if I want to multiply by 500, I just need to multiply by a thousand - 933.2 - and halve it - which is 466.6. It’s hardly rocket science.”

The student nodded, and thought. “So, if you wanted to divide by 500, you could divide by 1000 and then… double?”

The Mathematical Ninja nodded.

The student’s eyes widened. “You could do almost the same thing with 25s!”

Another nod.

“Even things like 33… although you’d be a little way off?”

The Mathematical Ninja, finally smiled. “We know a way to adjust for that.”

## A selection of other posts

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