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Option 3 : 36
P can do a piece of work in 20 days
Part of work P can do in a day = 1/20
Q can do a piece of work in 30 days.
Part of work Q can do in a day = 1/30
∴ Part of work P and Q can do in 5 days together = 5/20 + 5/30 = 5/12
∴ Remaining part of work = 1 - (5/12) = 7/12
R finishes 7/12th part of the work in 21 days
∴ R can complete the whole work alone in 21/(7/12) = 36 days
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