Saurabh started to make a fence around his farm. After he completed half of the fence, Basant started to break his fence every night, due to which his work got delayed by 20 days. If Basant did not interrupt the work, Saurabh would have completed his fence in 20 days. Find in how many days Basant can break this whole fence alone?

Saurabh would complete the fence in 20 days without interruption. After completing half the work, Basant starts breaking it every night, delaying completion by 20 days (so total 40 days). In those 40 days, Saurabh works 40 days at rate R, Basant breaks 30 nights at rate B. Net work: 40R - 30B = 20R (total fence). Solving: 20R = 30B, so B = 2/3 R. Since total work is 20R and Basant's rate is 2/3 R, time = 20R / (2/3 R) = 30 days. Option D matches.