P and Q together can do a work in 6 days. If P alone can do it in 15 days. In how many days can Q alone do it?  Choose the correct choice from the following answers.
A) 5 B) 4 C) 10 D) 11 E) 12

Rationale: 1 / 6 - 1 / 15 = = > 1 / 10 = = > 10 answer c
Answer: C