Working at a constant rate, P can finish a job in 4 hours. Q, also working at a constant rate, can finish the same job in 20 hours. If they work together for 3 hours, how many more minutes will it take P to finish the job, working alone at his constant rate?  Choose the correct choice from the following options
A) 12 B) 15 C) 20 D) 21 E) 24
Each hour they complete 1/4 + 1/20 = 3/10 of the job.   In 3 hours, they complete 3(3/10) = 9/10 of the job.   The time for P to finish is (1/10) / (1/4) = (2/5) hour = 24 minutes   The answer is 24.
<result>E</result>
Working at a constant rate, P can finish a job in 4 hours. Q, also working at a constant rate, can finish the same job in 20 hours. If they work together for 3 hours, how many more minutes will it take P to finish the job, working alone at his constant rate?