I dont think the math is very time consuming... You just need to practice using long division and multiplication tables alot, soon enough you will be able to to them in your head.
Some tips are to round numbers for easy calculating, for the Gas constant R. dont use the .0820574 instead use .82 the 4 possible answers will usually be of enough difference from one another that some rounding errors will not affect your answer.
Also try estimating what the asnwer could possibly be, for instance if one answer is 15M and another is .15M, Chances are it is the .15M because basic chemistry states that 15M solutions are very rare and probably would not be ecountered often. Using common sense can reduce the time it takes to answer a question...