Discussion

Explanation:

C’s efficiency = 1/20

C’s efficiency is half that of A and B combined.
⇒ A’s efficiency + B’s efficiency = 1/10

A is 50% as efficient as B or B’s efficiency is twice that of A.

⇒ A’s efficiency + 2 × A’s efficiency = 1/10

⇒ A’s efficiency = 1/30

⇒ Time taken by A alone to complete the work = 30 days.
∴ Time taken by B alone to complete the work = 15 days.

Now, let Time taken by A, B and C together = N days
⇒ $\frac{1}{N}$ = $\frac{1}{30}+\frac{1}{15}+\frac{1}{20}$ = $\frac{2+4+3}{60}$ = $\frac{9}{60}$ = $\frac{3}{20}$

⇒ N = 20/3 = $6\frac{2}{3}$ days.

Hence, option (b).

» Your doubt will be displayed only after approval.