P and Q together can do a job in 6 days. Q and R can finish the same job in 60/7 days. P started the work and worked for 3 days. Q and R continued for 6 days. Then the difference of days in which R and P can complete the job alone is:
(a) 10 days
(b) 8 days
(c) 12 days
(d) 15 days