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

A) 30
B) 35
C) 38
D) 40

Explanation:

Assume that in $x$ days, one cow will eat one bag of husk.

More cows, fewer days (Indirect proportion)
More bags, more days (direct proportion)

Hence we can write as

$\begin{array}{cc}\text{(cows)}& 40:1\\ \text{(bags)}& 1:40\end{array}\}::x:40$

$\Rightarrow 40\times 1\times 40=1\times 40\times x\Rightarrow x=40$