A has to pay £51 (and B pays £34)
This is how you work it out: A:B
3:2
3+2=5, 85/5 = 17 (this is what you would have to pay if your part of the ratio was 1)
therefore, to find what A pays, you times the part 1 by 3 (as the ratio is 3:2)
17 x 3 = 51
(likewise, 17x2= 34 which is what B has to pay)