( P ) and ( Q ) can complete a piece of work in 20 days, ( Q ) and ( R ) in 15 days, and ( P ) and ( R ) in 12 days. In how many days can ( P ), ( Q ), and ( R ) respectively complete the work if they work individually?
a) 30 days, 45 days, 60 days
b) 40 days, 50 days, 60 days
c) 25 days, 30 days, 40 days
d) 36 days, 48 days, 60 days