Markscheme answer is 21/271 = 0.077 and 40/271 = 0.148, but I get 21/236 = 0.089 and 40/236 = 0.170. Am I being stupid or is the markscheme wrong?

I do not understand this question Will. Is some of the explanation missing?

Add up all the numbers to get 236. In row a1, column b2 is 21, so Probability is 21/236.

I get the same as you Will. Looks like whoever did the marking scheme added an extra 35 into the total!