If anyone caught BBC1’s consumer moanfest *Watchdog* this week, they may have been pleasantly surprised to see Aperiodicobber^{1} Matt Parker featured in the show. Following a segment about a UK sports chain and its shocking use of the classic ‘UP TO 70% OFF’ ruse, they invited Matt on the show to explain how to calculate percentages more easily, and so that Anne Robinson could mock him for being Australian, apparently.

Since the tips Matt presented were useful, we at the Aperiodical thought it was worth reproducing Parker’s Patented Percentage Ploys here, for your reference.

**1. If it’s a multiple of 10%, it’s easy to calculate**

Although it turns out it’s also easy to wind up school teachers – while Matt’s use of the phrase ‘you move the decimal point’ was instrumental in his explanation of how to find 10% of something easily, it also sparked off several annoyed tweets from primary school teachers, who seem to be striving to eliminate use of the phrase and instead insist that it’s the *DIGITS* that move, and not the point. While I’m sure a large proportion of the nation’s 10-year-olds were avidly tuned in to this consumer rights show, hopefully they have managed to survive unconfused.

I’ll move decimal points if I want to. I’m a professional.

— Colin Beveridge (@icecolbeveridge) May 14, 2014

**2. You can swap the percentage and the amount**

In the example Matt gave, 16% of £25 is difficult to calculate in your head, but it turns out that 25% of £16 is precisely the same amount. While not necessarily news to everyone, this is probably the top tip which will have had the most impact – everyone I’ve mentioned it to since has a) been amazed and b) commented on how useful that will be from now on.

**3. Close enough is good enough**

Rather than just demonstrating Matt’s advocacy of rounding things, this simple trick is quite nice – if you want to calculate, for instance, 22% of £48, you can take the most significant figure of each (rounded to the nearest ten), so 2 and 5, and multiply them together to get £10. This is within 56p of the correct answer, and will almost always give something roughly right, if you’re in a hurry. Our only criticism of Matt’s explanation was that he forgot to mention this only works for 2-digit percentages and 2-digit numbers, and you’d need to adapt the method to work for other types of number. Still – a time-saver!

### More information

Watch the episode on the BBC iPlayer (segment starts at 38m30s; UK only, and it will stop working at some point)

Matt’s Twitter page, where you can see some of the fall-out.

BBC Watchdog programme page

- The internet assures me that ‘cobber’ is Australian slang for ‘friend’. [↩]