A lamp rated $100 W$, $240 V$ is lit for $5$ hours. Calculate the cost of lighting the lamp if $1 kWh$ of electrical energy cost $₦5$.

If a $100 W$ lamp is lit for $5$ hours, $5 \cdot 100 = 500 Wh$ of electricity will be used. To convert $500 Wh$ of energy to $kWh$, you divide by $1000$. We now have that $\frac{500}{1000} = 0.5 kWh$ of energy is used. Since $1 kWh$ of electricity costs $₦ 5$, $0.5 kWh$ will cost $0.5 \cdot 5 = ₦ 2.5$.