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?

Let the required number of days be x.

Less cows, More days (Indirect Proportion)

Less bags, Less days (Direct Proportion)

$cows 1:40 :40:xbags 40:1 :40:x$

1 x 40 x x = 40 x 1 x 40

=>  x = 40.