P, Q and R together can make a table in 40 minutes. P and Q together can make it in 60 minutes. How much time (in minutes) will R alone take to make the table?

Let total manwork required to build the table = L.C.M. (40,60) = 120 units

P, Q and R together can make the table in 40 minutes.

=> Total efficiency of P, Q and R together = $$\frac{120}{40}=3$$ units/min

Similarly, (P+Q)'s efficiency = $$\frac{120}{60}=2$$ units/min

=> R's efficiency = $$3-2=1$$ unit/min

$$\therefore$$ Time required by R alone to build the table = $$\frac{120}{1}=120$$ minutes

=> Ans - (B)