Thank you so much for pointing that out. I was having trouble finding the category theory book but finding the Pi one by the same author everywhere!

If not in any particular rush, you can get a UK copy of 'How to Bake Pi' at http://www.bookdepository.com/ (best price, free shipping, and they take paypal). Plus it turns out that Eugenia Cheng just released another book in UK on June 4 called 'Cakes, Custard and Category Theory: Easy Recipes for Understanding Complex Maths' which seemed interesting so that was ordered as well. Shipping time to US is listed as 5 days.

Plus if you need a .pdf of Feynman's favorite math book (Calculus for the Practical Man - Thompson (1946)) currently there are copies at KAT and PB.

Not if this is considered a particularly difficult UK Maths GCSE exam question, which over 90 percent couldn't answer:

Hannah has 6 orange sweets and some yellow sweets. Overall, she has n sweets. The probability of her taking 2 orange sweets is 1/3. Prove that: n^2-n-90=0.

If a HS student I would imagine you would

First: think of coins p(H1) = .5; p(H2) =.5; p(H1+H2) = .5 * .5 = .25

Second: OK, here (6/n) * (5/(n-1)) = 1/3

Third solve: 30/(n * (n-1)) = 1/3

                     90 = n^2 - n
                      0 = n^2 - n - 90

n=10 or -9; if -9 then Hannah only has one sweet and lifted 9 from somebody else just to demo her point.

(from London Telegraph today)

    C(6,2) / C(n,2) = 1/3

    15 / [n(n-1)/2] = 1/3
    30 / n(n-1) = 1/3
    n(n-1) = 90