Lornz can finish checking a set of papers in 2/3 hours as Butch can and Butch can do it in 3/4 as many hours as Fe. working together, they can do it in 36/13 hours. In how many hours can each of them alone, finish checking the set?
Basically,
Problems involving rates, the key to solving these typical, or 'prosaic' type of questions is to find the individual rates. The sum of the individual rates should equal the total rate.
Jhevon brilliantly shows this reasoning though his mathematical steps.
Apply this principle to the commonplace questions, and you will master them. Good luck.