Anil and Suman together can do a work in 12 days, which Anil alone can do in 20 days. If Suman alone has to do this work, he will take

Anil and Suman complete the work together in 12 days (rate = 1/12 per day), and Anil alone completes it in 20 days (rate = 1/20 per day). Suman's one-day work rate is 1/12 - 1/20 = (5 - 3)/60 = 2/60 = 1/30. Thus, Suman alone will take 30 days to finish the entire work.