How many planks of dimensions (5 m × 25 cm × 10 cm) can be stored in a pit which is 20 m long,

(a)  480

Length of the pit = 20 m
Breadth of the pit = 6 m
Height of the pit = 50 cm = 0.5 m

Length of the plank = 5m
Breadth of the plank = 25 cm = 0.25 m
Height of the plank = 10 cm = 0.1 m

$\therefore$ Number of planks $=\frac{\text { volume of the pit }}{\text { volume of } 1 \mathrm{plank}}$

$=\frac{20 \times 6 \times 0.5}{5 \times 0.25 \times 0.1}$

$=\frac{60 \times 1000}{125}$

$=480$