A contractor agreed to finish a job in 30 days, with a penalty of ₹2,500 for each day beyond that. He put 24 men on it, and at the end of the 20th day exactly half the job was done. He then brought in 8 more men of the same skill and the job ran on to its finish. What penalty does he pay?

In 20 days 24 men put in 480 man-days for half the job, so the whole job is 960 man-days and 480 man-days remain. With 32 men that takes 480 ÷ 32 = 15 days, so the job runs 20 + 15 = 35 days — five days past the agreed 30. The penalty is $5\times 2,500$ = ₹12,500. ₹25,000 keeps the original 24 men for the closing stretch (480 ÷ 24 = 20 days, ten days late). ₹37,500 charges the penalty for all 15 days of the closing stretch instead of only the 5 beyond the deadline. ₹50,000 makes both errors at once, charging 20 late days.