The Official Guide For GMAT® Quantitative Review, 2ND Edition
If y is an integer, is y^3 divisible by 9 ?
(1) y is divisible by 4.
(2) y is divisible by 6.
takind A alone, if y is divisible by 4 then y= 4n therefore , y^3 = (4n)^3 = 64n^3 . thus we cant say anything about the divisibility by 9 since it will not depend totally on the value of n
taking B alone, if y is divisible by 6 then y= 6n therefore , y^3 = (6n)^3 = (2*3*n)^3 = (8* 9*3) n^3. thus we can say that it is divisible by 9.