In a dairy farm, 20 cows eat 20 bags in 20 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 * 20 * x = 20 * 1 * 20

x = 20.