Posted in ninja maths.

Countdown puzzle: Make 952 from 100, 75, 50, 25, 6 and 3 Have a go at this one before you read on. It's pretty straightforward to make 953 (exactly as the poor antagonist, Gerald, in this story does): 100 × (6 + 3) = 900 900 + 50 = 950

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Posted in ninja maths.

Countdown puzzle: Make 952 from 100, 75, 50, 25, 6 and 3 Have a go at this one before you read on. It's pretty straightforward to make 953 (exactly as the poor antagonist, Gerald, in this story does): 100 × (6 + 3) = 900 900 + 50 = 950

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Posted in ninja maths, probability.

Million-to-one shots come up nine times out of ten. - Terry Pratchett While researching rules for ninjas to live by (don't ask), I came across a mathematical phenomenon I'd never noticed noticing. It's this: The Law of Inverse Ninjas: The probability of a group of ninjas winning a battle in

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Posted in ninja maths.

Your average ninja (if there is such a thing) doesn't need to do the swirly-sword manoeuvre. If he wanted, he could just run you through and you'd be dead before you even noticed he was there. But that's rather rude and unsophisticated, the kind of thing a pirate might do.

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Posted in ninja maths.

Learning the rough value of a few key numbers worth knowing can make ninja maths a lot more impressive later on - especially if you know how roughly how rough the rough values are. A little bit about the tables below: I'm giving you decimals to two sig figs, the

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Posted in ninja maths.

Difficulty: ** Impressiveness: **** If you do A-level maths, you do an awful lot of integration. You integrate polynomials, trig functions, partial fractions, exponentials, parametric curves, products of these... and get nice analytical answers. Here, let me provoke controversy: Unless you're going to be a pure mathematician or a maths

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Posted in ninja maths.

Difficulty: (simple version) *** (advanced version) ***** Impressiveness: ***** Accuracy: *** If you're a statistician, you quite often end up working out powers of numbers just a little less than 1. What's the probability of rolling a pair of dice ten times and never getting a double six? It's $\left(\frac{35}{36}\right)^{10}$.

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Posted in ninja maths.

Some of the secrets of the mathematical ninja are pretty pointless, when you come down to it: after all, we have machines for most of these things. The divisibility tricks are useful (as far as I can see) only in a very specific circumstance: when you're deciding whether to cancel

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Posted in ninja maths.

Difficulty: ** Impressiveness: **** (Many thanks to Swar for pointing me at this one - and challenging me to explain it well!) It's surprisingly easy to square numbers near 50. Here's the recipe: 1. Find the difference between your number and 50. (If you're looking at 46, it'd be -4.

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Posted in ninja maths.

The trick: someone says 'what's 7.5 squared?' and - mentally squaring in a flash - you say: 56.25. Squaring halves Squaring halves is really easy if you know your times tables. Here's the method: Take your number and find the whole numbers immediately above and below. If you're trying to

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