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Triple the initial number, add a zero at the end, now halve the resulting number is good enough for my purpose.

Brain is surprisingly good at doubling, tripling and halving operations.

And visually added zero is quite easy too.

   *3, *10, /2 == *15 
I think you forgot a /10 at the end?

I'm surprised this is faster than adding half of the original number.

That converts miles to approximate hectometres.

It's within 10% error.

Yes, it's within 90%.

If you are going that route, why not multiply by 8 and divide by 5 for a much better approximation. But to be honest for me it has always been easier to directly multiply by 1.6 (or 1.5 for your case)

So 1 mile equals 15km? and then you wonder why all these units conversion and up with probe crashing on mars.

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