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On this month's episode of Wrong, But Useful, @icecolbeveridge and @reflectivemaths are joined by special guest co-host @christianp. This time, we talk about:

- Christian, who is involved in @mathsjam and the @aperiodical, and has a number of the podcast: 13. He dislikes it because of its times table; I like it because you can work out thirteenths in your head and look like a ninja, explained in three parts here: Part 1, Part 2, and Part 3. Dave likes 13 because of Belfigore's prime.
- Colin mistakes 91 for a prime (although immediately corrects himself). He would be lousy at Is This Prime?, which he reviewed here. Christian gives the top ten errors in the game. We discuss when 1 stopped being a prime.
- Talking of things people find difficult, Christian has found an article looking into the multiplications students get wrong. Christian mentions tropical arithmetic.
- Dave rants about why BIDMAS is awful. We mention Maths In The City
- Dave has a new calculator, which sounds like a submarine. Colin dislikes icosikaitetragons, when 24-gon is much clearer. Dave wants to rename the months from September onwards. We digress into how A-level curricula are made, and Christian suggests exam speed runs.
- Chris Staecker reviews calculating machines. Christian regrets not buying a Corta when he had the chance.
- Colin wants to know who the Osborn behind Osborn's law was. (Possibly George Osborn, who wrote a textbook with French – but he canâ€™t find much more information than that.)
- For IWD: how many maths books do you own that were written by women? More than Martin Gardner books? Are there any suffragette maths texts?
- Puzzles: @chrishazell72 gets a gold star for his answer to the previous puzzle of $\left(\frac{125}{216}\right)^6 + \left(\frac{91}{216}\right)^6 \approx 0.043$. This month, Christian has a puzzle without an answer: pick a random number between 0 and 1, then a random number between 0 and 2, and so on up to a random number between 0 and 1000. What is the expected mean of your 1000 numbers? Also, what are the chances of a 1-in-3 shot occurring ten times in a row?
- The Wrong, But Useful 4th anniversary is on March 28th. If any of our loyal listeners had a nice message to splice into our next episode by way of celebration

## Colin

Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008.
He lives with an espresso pot and nothing to prove.

## Jonathan Schwartz

I think the answer to the first puzzle is 250.25

http://i.imgur.com/OOqEIrU.jpg

## Colin

I haven’t worked it through yet – it’s certainly not implausible! Thanks đź™‚