A12.2-g sample of x reacts with a sample of y to form 78.9g of xy. what is the mass of y that reacted?

The chemical reaction would most likely be written as:

X + Y = XY

We cannot simply use the usual method of converting grams to moles since we do not have any idea on what are the identities of X and Y. The only method we could use is to use the law of conservation of mass where mass inflow in a process should be equal to the mass out in the process. The total inflow of mass would be the mass of X and Y and the outflow would be the product XY.  

g X + g Y = g XY
12.2 g + g Y = 78.9 g XY
g Y = 66.7 g 

It's a