i'm having huge issues with these word problems:

White vinegar is a solution of acetic acid in water. There are two strengths of white vinegar - a 5% solution and a 10% solution. How many millilitres of each solution must be mixed to make 50mL of a 9% vinegar solution?

the equations that i thought would work are:

x +y =50

0.05x + 0.10y = 0.09

but they don't.

can someone explain??