Kavita is 25% more efficient than Lalit. Working together they finish a stock-taking task in 20 days. How many days would Lalit need on his own?

If Lalit's daily output is L, Kavita's is 1.25L, so together they give 2.25L = 1/20 of the task a day. Then L = 1/45, and Lalit alone needs 45 days. 36 days is Kavita's solo figure (1.25/45 = 1/36) — the quicker worker, not the one asked for. 25 days adds 25% to the joint 20 days; the percentage attaches to output, not to the pair's time. 16 days shrinks the joint 20 days by the same factor and is impossible, since neither can beat the pair.