In this type of proportion, when one quantity is increasing, the other is decreasing and vice versa.
Ex: a) 5 men built a house in 10 days. How long will take 10 men if they are working at the same rate?
b) 15 men hoe a piece of land in 120 days, how long will it take 8 men working at the same rate?
Solution: a) If 5 men = 10 days, 10 men = (5 * 10) / 10 = 5 days. So, 10 men will take 5 days to build the house.
b) If 12 men = 120 days, 1 man = 12 * 120 = 1440 days. Therefore;
8 men = 1440 / 8 = 180 days
