Raju is thrice as good as a workman as Vinod and together they can finish a task in 21 days. In how many days can Vinod alone complete the work?

Correct option is B

Given:

Raju is three times as good a workman as Vinod.

Together, Raju and Vinod can complete the task in 21 days.

We need to find the number of days in which Vinod alone can complete the work.

Solution:

The total rate of work done by both Raju and Vinod together is the sum of their individual rates:

​$$\frac{1}{v} + \frac{3}{v} = \frac{1}{21}$$​

​$$\frac{1 + 3}{v} = \frac{1}{21}$$​

​$$\frac{4}{v} = \frac{1}{21}$$​

​$$4 \times 21 = v$$​

v = 84

Vinod alone can complete the work in 84 days.

Alternate Method:

Work done by Vinod = $$\frac{84}1=84$$​​