P is 10% more efficient than Q. P can complete a work in 21 days. If P and Q work together, how many days will it take to complete the same work?	Choices:
A) 7  B) 9  C) 11  D) 13  E) 15

Rationale: "the work done by p in 1 day = 1 / 21 let work done by q in 1 day = q q × ( 110 / 100 ) = 1 / 21 q = 100 / ( 21 × 110 ) = 10 / ( 21 × 11 ) the work done by p and q in 1 day = 1 / 21 + 10 / ( 21 × 11 ) = 21 / ( 21 × 11 ) = 1 / 11 p and q together can do the work in 11 days . the answer is c ."
Answer: C