Explanation:

Let the time taken by Sohan to complete the work = x days
Then, time taken by Rohan to complete the work = 2x days and
time taken by Mohan to complete the work = 4x days
According to the question:
12 (1/x + 1/2x + 1/4x) = 1
⇒ 4 + 2 + 1 /4x = 1/12
⇒ 7/4x = 12
⇒ x = 21 days
Hence, time taken by Sohan to complete the work = x = 21 days
time taken by Rohan to complete the work = 2x = 2 x 21 = 42 days
time taken by Mohan to complete the work = 4x = 4 x 21 = 84 days
let the no of days taken by Rohan and Sohan to complete the work = p days
According to the question:
⇒ p (1/21 + 1/42) = 1
⇒ p 1+2/42 = 1
⇒ p = 14 days