A, B and C working separately can do a piece of work in 11 days, 20 days and 55 days respectively. In how many days, the work will be completed if A is assisted by B and C on alternate days ?

Let total work to be done = L.C.M.(11,20,55) = 220 units

A can complete it in 11 days, => A's efficiency = $$\frac{220}{11}=20$$ units/day

B's efficiency = $$\frac{220}{20}=11$$ units/day

and C's efficiency = $$\frac{220}{55}=4$$ units/day

If A is assisted by B and C on alternate days, => work done on each day = (A+B), (A+C), (A+B), (A+C), .....

=> Work done in 2 days = (20+11) + (20+4) = 55

Number of days required = $$\frac{220}{55}\times2=8$$ days

=> Ans - (D)