If you had 1 quart and you now add 2 quarts, the total now becomes 3 quarts of water/juice mix.
So the first solution contains 1/2 quart of juice and 1/2 quart of water. Half of the first mixture and then two more parts now results in 2.5 parts of juice and half part water.
Remember its now a total of 3 parts. Therefore the juice in the mix is now;
The final mix therefore contains 83.33% juice