The table on the £50 note is from Alan Turing’s paper “On Computable Numbers, with an Application to the Entscheidungsproblem”, in which he showed that some purely mathematical problems can never be answered by computation.

Hopefully you’ll find that the puzzle below is not such a problem!

An hypothetical conversation between the shades of Hugh Alexander and Alan Turing:

Solution

The sequence is 20, 8, 2, 10, 3, 7

20 + 8 + 2 + 10 + 3+ 7 = 50
£10 and £20 are banknotes
2, 3, 7 are prime
2 < all other numbers
20 > 10 + 8
10 cannot follow 20
20 + 2 + 3 = 10 + 7 + 8

You can then “apply” your answer to the word “Entscheidungsproblem” itself. So take the 20th character, then the 8th, etc.The answer is MINUTE.