Assume that contents of the container is X 

On the first day 1/3rd is evaporated. 

(1 - 1/3) of X is remaining i.e. (2/3)X 

On the Second day 3/4th is evaporated. Hence, 

(1- 3/4) of (2/3)X is remaining 

i.e. (1/4)(2/3)X = (1/6) X 

Hence 1/6th of the contents of the container is remaining