A certain number of men can complete a piece of work in 6k days, where k is a natural number. By what percent should the number of men be increased so that the work can be completed in 5k days?

Let the original number of men be M, completing the work in 6k days. Total work = M × 6k man-days. To complete the same work in 5k days, let the required number of men be M'. Then M' × 5k = M × 6k, so M' = (6/5)M = 1.2M. The increase is 0.2M, which is 20% of M. Hence the number of men should be increased by 20%.