A 12 HP motor has to be operated 8 hours/day. How much will it cost at the rate of 50 paisa/kWh in 10 days

If a motor of 12 HP works for 10 days at the rate of 8 hr/day then energy consumption = power × time 

$=12\times 746\frac{\mathrm{J}}{\sec }\times (80\times 60\times 60)\sec$

$=12\times 746\times 80\times 60\times 60\mathrm{J}=2.5\times {10}^9\mathrm{J}$

$\text{ Rate of energy }=50\frac{\text{ paisa }}{\mathrm{kWh}}$

$\text{ i.e. }3.6\times {10}^6\mathrm{J}\text{ energy cost }0.5\mathrm{Rs}$

$$\begin{gathered} \text{ So }2.5\times {10}^9\mathrm{J}\text{ energy }\mathrm{cost}=\frac{2.5\times {10}^9}{2\times 3.6\times {10}^6}=347\mathrm{Rs} \\ NEARER option Rs.350/- \end{gathered}$$