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? Select the correct option.
A) 12  B) 15  C) 20  D) 21  E) 24

Rationale: "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 e ."
Answer: E