Two identical beakers are filled with acid solutions, the ratio of the volume of acid to the volume of water being p:1 in one beaker and q:1 in the other beaker. If the entire contents of the two beakers are mixed together, the ratio of the volume of acid to the volume of water is:
This looks nasty, but here we go.
If the ratio of acid to water is p:1, then if the beaker contains x cc of liquid total, then 
is acid and 
is water.
It’s getting started that’s hard.