(First, if this is in the wrong section, im sorry!)

Alright so here is the word problem.

A bar of metal contains 10% Silver.

Another bar of metal contains 15% Silver

How many kg of each is required to make a 10kg bar of 12% Silver?

The only problem, is that the teacher stated you can only use ONE variable in your equation.

I am entirely lost on this one...If i can use 2 variables, I can do that...but one?