P is three times as fast as Q and working together, they can complete a work in 15 days. In how many days can Q alone complete the work?  Choose the correct choice.
A) 16 days  B) 17 days  C) 19 days  D) 14 days  E) 20 days
P = 3Q   P + Q = 3Q + Q = 4Q   These 4Q people can do the work in 15 days, which means Q can do the work in 60 days.   Hence, P can do the work in 20 days.   Answer:20 days
<result>E</result>
P is three times as fast as Q and working together, they can complete a work in 15 days. In how many days can Q alone complete the work?