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?

Answer: Option C

Let the required number of days be x.
Less cows, More days (Indirect Proportion)
Less bags, Less days (Direct Proportion)
$$\begin{array}{c}\text{Cows}1:40\text{hfill}\\ \text{Bags}40:1\text{hfill}\end{array}\}::40:x$$
$$\begin{array}{rl}& \therefore 1\times 40\times x=40\times 1\times 40\\ & \Rightarrow x=40\end{array}$$