Srinivas can do a job in 15 days and Ramesh can do the same job in 9 days. With the help of Ravi, they did the job in 3 days. In how many days can Ravi alone do the same job?

Correct option is B

Given:

Srinivas = 15 days.

Ramesh = 9 days

With the help of Ravi = 3 days.

Calculation:

Let Ravi can do job in x days.

Job done by Srinivas in one day = 1/15

Job done by Ramesh in one day = 1/9

Job done by Ravi in one day = 1/x

As per the question,

​$$\frac{1}{15} + \frac{1}{9} + \frac{1}{x} = \frac{1}{3} \\\ \\\frac{1}{x} = \frac{1}{3} - \frac{1}{15} - \frac{1}{9} \\\ \\\frac{1}{x} = \frac{15 - 3 - 5}{45} \\\ \\\frac{1}{x} = \frac{7}{45} \\\ \\x = \frac{45}{7} \\\ \\x = 6 \frac{3}{7}$$ 

Then Ravi can do this job in  $$6\frac{3}{7}$$ days.​