A courtyard is 25 m long and 16 m broad is to be paved with bricks of dimensions 20 cm by 10 cm. What is the total number of bricks required ?

Answer: Option C

Given length and breadth of courtyard is 25 m long and 16 m respectively
Area of courtyard :
$$\begin{array}{rl}& =\left(25\times 16\right)sq.m^2\\ & =400sq.m^2\end{array}$$
Dimensions of bricks 20 cm by 10 cm
Area of the surface of brick $$=\left(\frac{20\times 10}{10000}\right)m^2$$
∴ Number of bricks :
$$\begin{array}{rl}& =\frac{400}{\frac{20\times 10}{10000}}\\ & =\frac{400\times 10000}{20\times 10}\\ & =20000\text{ bricks}\end{array}$$