A lamp rated 100W, 240V is lit for 5hours. Calculate the cost of lighting the lamp if 1kWh of electrical energy cost N5.

Analyze the question: A 100 W lamp runs for 5 hours. Electrical energy costs N5 per kWh. We need the total cost.

Concept: Electrical energy used = Power × Time. For billing, energy is measured in kilowatt-hours (kWh):

Energy (kWh) = Power (kW) × Time (hours)

Cost = Energy (kWh) × Price per kWh

The 240 V rating tells us the voltage the lamp is designed for. It is not needed here because the power rating is already given.

Solution:

1. Convert power to kW: 100 W = 100 ÷ 1000 = 0.1 kW

2. Calculate the energy used: Energy = 0.1 kW × 5 h = 0.5 kWh

3. Calculate the cost: Cost = 0.5 × N5 = N2.50

Answer: The cost is N2.50, which is option A.

Tip: In "cost of electricity" questions, follow this routine: Power (kW) × Hours = kWh, then kWh × Price = Cost. Watch out for distractors like the 240 V here. If the power is already given in watts, you don't need the voltage. Also, always convert W to kW before multiplying, since forgetting this is the most common mistake.