black keys : white keys
b : w = 36 : 52
b : w = 18 : 26 = 9 : 13
b : w = 9 : 13
Total ratio = 9 + 13 = 22.
It means for every 9 black keys you have 13 white keys.
It also means for the piano the fraction of black keys is 9/22 of the entire piano
It also means for the piano the fraction of white keys is 13/22 of the entire piano