The Question is

The answer is 12 and i get 10Alloy of copper and tin is 20% copper , how many pounds of copper must be added to 20 pounds of the alloy in order for resulting alloy to be 50% copper

Here is how i constructed the table (What am i doing wrong)

Pounds Percentage Amount of solvent Component 1 x 20%=0.2 0.2x Component 2 20-x 80%=0.8 0.8(20-x) Mixture 20 50/100 0.2x+0.8(20-x)=20.0.5

This is the equation that i get : 0.2x+0.8(20-x)=20.0.5

Could anyone please tell me what i am doing wrong ??