How many quarts of pure antifreeze must be adddd to 9 quarts of a 10% antifreeze solution to obtain a 20% antifreeze solution?

Let x be the amount of pure antifreeze to be added.

If we add this to the 9 quarts, how much do we have? 9+x quarts. We are told this must be 20%.

So, we have

x+.10(9)=.20(x+9)

Solve for x.