In a dairy farm, 40 cows eat 40 bags of husk in 40 days. In how many days one cow will eat one bag of husk ?

Assume that in x days, one cow will eat one bag of husk.
More cows, less days (Indirect proportion)
More bags, more days (Direct proportion)
Hence we can write as

• 1 x 40 x x = 40 x 1 x 40

x = 40.