Hello student,

I hope you are doing well.

In compound B, 32 parts of X react with 84 parts of Y,

=> In compound B, 16 parts of X react with 42 parts of Y,

In compound C, 16 parts of X react with x parts of Y,

The ratio of masses of Y which combine with fixed mass of X in compounds B and C is 3:5

B     |     42         3

C     |      x          5

=> x = (42X5)/3 = 70

So, the answer is 70 parts.

I hope it helps.