1. First you have to divide the total number of keys by the parts that the ratio contains:
88/(13+9) = 88/22 = 4
2. This number is how many keys are in 1 part of the ratio so you have to times this number by the number in the ratio to find out how many keys there are in that section:
13 x 4 = 52 (there are 52 white keys)
9 x 4 = 36 (there are 36 black keys)
3. To check your answer, ad up both parts of the ratio!
52+36 = 88
Hope this helps :)