Method 1: Using man-days
Amount of work in terms of man days =24×20=480 man days
So, number of men required to complete the work in 12 days =48012=40 men.
16 more men are required. Hence option (c)
Method 2: Using constant product rule
Decrease in number of days =1(208)=12.5
Increase in number of men =11.5= 16 men to finish the same work.