What is the cost of running five 50W lamps and four 100W lamps for 10 hours if electrical energy costs 2 Kobo per KWh?

Analyze the question: We need the total energy used (in kWh) by all the lamps over 10 hours, then multiply by the cost per kWh.

Concept:

• Energy (kWh) = Power (kW) × Time (hours)
• Cost = Energy (kWh) × Rate

Solution:

1. Power of the five 50 W lamps: 5 × 50 = 250 W
2. Power of the four 100 W lamps: 4 × 100 = 400 W
3. Total power: 250 + 400 = 650 W = 0.65 kW
4. Energy used in 10 hours: 0.65 × 10 = 6.5 kWh
5. Cost at 2 kobo per kWh: 6.5 × 2 = 13 kobo

Converting to naira (100 kobo = N1): 13 kobo = N0.13

Check against the options: N0.

Answer: D (N0.13)

Tip: Always convert watts to kilowatts first (divide by 1000), and watch the units at the end. The rate is in kobo, so convert to naira by dividing by 100. Distractors like N0.65 and N3.90 come from skipping or misapplying these conversions.