Working alone, three persons, P, Q and R can do a certain job

Working alone, three persons P, Q, and R can do a certain job in 10, 12, and 15 hours respectively. What is the ratio of the maximum and minimum time taken to complete the job, if at every hour, at least 1 person must work?

A.1:3
B.1:2
C.1:1
D.3:1
E.15:4

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