I actually drew the jars to solve it, without calculating anything...
Just to give you a good laugh, I will describe how:
So, I first created one jar, and marked its 1/4th and 1/3rd. This showed me that if these were 2 jars, the one filled up to its 1/4th must be the bigger one.
Then I just drew 2 same jars, next to eac other, and marked 1/4th in one and 1/3rd in the other. I pulled the line that marked the 1/4th of one jar on the second jar, and saw that there was only a little it lower than the 1/3rd. The I realised that if I moved the rest to jar one, it would be filled half way... So, 1/2.
Mind that I am a psychologist... Sometimes, it is easier for me not to do the math!