The transparent window on the £50 note is based on a pattern detailing one of the huts at Bletchley Park.

We found an encoded message scribbled under one of them.

Give it sum thought, and see if you can make it add up.

If you look at it from a different angle you should see a Welsh town through the window, which would be a real plus!

Solution

This one is not massively clear despite all the hints that you need to add numbers up. The key to figuring it out is that the numbers at the bottom are sums of all the number in the column above (note that D appears twice).

There are only five different shades in the puzzle so each must refer to a one of five numbers. It’s then simple deduction to work out the value of each shade.

2 + 2 = 4
2 + 6 = 9
1 + 3 = 4
4 x 2 + 2 x a = 22 therefore a = 7
7 + 8 = 15

Then the next clue is that the two question marks refer to a sum and a letter. It’s not clear why they have alternated the colours in each row.

Summing the rows gives the following:

And it’s trivial to see that each letter’s value is its ordinal position in the alphabet, i.e. A = 1, B = 2, C = 3, etc.

The answer is CORWEN.