A contractor under takes to make a road in 40 days and employs 25 men. After 24 days, he finds that only one third of a road is made. How many extra men should he employ so that he is able to complete the work 4 days earlier?

In 24 days, 25 men complete 1/3 of the work. So 25 men × 24 days = 600 man-days for 1/3 work, meaning total work = 1800 man-days. Remaining work = 1200 man-days. Time remaining = 40 - 24 - 4 = 12 days (4 days earlier than scheduled). Men needed = 1200/12 = 100 men. Extra men = 100 - 25 = 75 men. This follows the chain rule: men × days is proportional to work.