chem..

I assume you mean

"compound X2Y is 60% X by mass, calculate the percent Y by mass of the compound X2Y2 "

If that is so then X:Y = 3:4 (which is also true for X2:Y2)

So %Y =4/7 x 100%= appx 57.14%

(If on the other hand X2Y was 60% Y by mass, then similar reasoning would give %Y=75% in X2Y2)