P cando a work in 15 days and Q cando the same work in 20 days. If they can work together for 4 days, what is the fraction of work left?  Answers:
A) 6 / 15
B) 7 / 13
C) 8 / 15
D) 9 / 17
E) 2 / 13

Rationale: amount of work p can do in 1 day = 1 / 15 amount of work q can do in 1 day = 1 / 20 amount of work p and q can do in 1 day = 1 / 15 + 1 / 20 = 7 / 60 amount of work p and q can together do in 4 days = 4 × ( 7 / 60 ) = 7 / 15 fraction of work left = 1 – 7 / 15 = 8 / 15 c
Answer: C