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 ?

Given length and breadth of courtyard is 25 m long and 16 m respectively
Area of courtyard :
$$\eqalign{ & = \left( {25 \times 16} \right)sq.{m^2} \cr & = 400\,sq.{m^2} \cr} $$
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 :
$$\eqalign{ & = \frac{{400}}{{\frac{{20 \times 10}}{{10000}}}} \cr & = \frac{{400 \times 10000}}{{20 \times 10}} \cr & = 20000\,{\text{ bricks}} \cr} $$