Maths riddle, try to solve

[McGherkin]

9 + 0.841 = 9.841
9.841 + 9 = 18.841

X=9
Y=0.841
Z=9.841

However, X and Y could be any two numbers that add up to 9.841. For example:

5 + 4.841 = 9.841
9.841 + 5 = 14.841

X= 5
Y= 4.841

So I don't see how there can be one solution to this problem.