Can anyone please help me with this chemistry question?

[Chiyuki_Snow]

Elements in Compound/Masses of Elements

X and Y---------------X=0.4g... Y=4.2 g

Y and Z---------------Y=1.4 g, Z=1.0g

X and Y---------------X=2.0g... Y=7.0 g

 

a. What are the assumptions in solving this problem?

b. What are the relative masses of X,Y, and Z?

c. What are the chemical formulas of the three compounds?

d. If you decompose 21g of compound XY, how much of each element is present?

 

for A, I think compound YZ contains twice as much element Y than compound XY. Also, compound XY contains 5 times more element X than compound XY

for B, Do I just have to add the masses?

for C, I don't know how to do this one.

for D, I'm thinking I have to solve C to solve this. If not my answer would be 16g of Y and 5 g of X.