A and B together can complete a task in 20 days while B alone can complete the same task in 60 days. A alone will be able to complete the same task in:

Correct option is C

Given:

A and B together can complete the task in 20 days

B alone can complete the task in 60 days

Solution:

Work/day by A =$$\frac{1}{20} - \frac{1}{60} = \frac{3 - 1}{60} = \frac{2}{60} = \frac{1}{30}$$​

Time taken by A alone =$$\frac{1}{\frac{1}{30}} =$$​30 days

Alternate Method:

Time taken by A alone = $$\frac{60}{3-1}=\frac{60}2 =30$$ days​