A house has ten 40W and five 100W bulbs. How much will it cost the owner of the house to keep them lit for 10 hours if the cost of a unit is 5N?

Analyze the question: We need the total energy used (in kWh, i.e. "units") and then multiply by the cost per unit.

Concept: Electrical energy = Power × Time. One unit = 1 kWh.

Solution:

1. Power of the ten 40 W bulbs: 10 × 40 = 400 W
2. Power of the five 100 W bulbs: 5 × 100 = 500 W
3. Total power: 400 + 500 = 900 W = 0.9 kW
4. Energy used in 10 hours: 0.9 kW × 10 h = 9 kWh (9 units)
5. Cost: 9 × N5 = N45

Answer: Option C (N45).

Tip: Always convert watts to kilowatts before multiplying by hours. Then cost = kWh × price per unit. Option A (N90) is what you get if you double the energy by mistake, so check your arithmetic and units.