The man completes 7/8 of the work in 21 days, meaning he completes 1/8 of the work in 3 days. The remaining 1/8 of the work would take him 3 days. If the quantum of work increases by 50%, the new total work is 150% of the original work. Since 7/8 of the work is done, 1/8 remains. The initial remaining 1/8 would take 3 days. A 50% increase means the total work becomes 1.5 units if the original was 1 unit. If 7/8 of the original is done, 1/8 is left. 50% of the original work is 50/100 * 1 = 0.5. This 0.5 work needs to be done in addition to the remaining 1/8. Total additional work is 1/8 + 0.5 = 0.125 + 0.5 = 0.625. Since 1/8 work takes 3 days, the rate is 1/24th of the work per day. To complete 0.625 work, it will take 0.625 / (1/24) = 0.625 * 24 = 15 days.