How many quarts of pure antifreeze must be added to 7 quarts of a 20% antifreeze solution to obtain a 50% antifreeze solution

The current 7 quarts of liquid contains 1.4 quarts of pure antifreeze at the current 20% antifreeze solution, This gives a 5.6 quart quantity of other liquid. In order to raise the antifreeze solution to 50% it is necessary to have equal amounts of pure antifreeze and other liquid in the total amount. Currently the other liquid quantity of 5.6 quarts is 4.2 quarts more than the amount of pure antifreeze currently included in the 7 quarts, therefore it is necessary to add another 4,2 quarts of pure antifreeze to the liquid. As a result, we end up with a total of 11.2 quarts of liquid at a 50% antifreeze solution.