Calculate total energy in kWh and multiply by unit cost

Steps:

• Total days: 3 weeks × 7 days/week = 21 days
• Total hours: 21 days × 5 hours/day = 105 hours
• Total energy: 100 W × 105 hours = 10,500 Wh = 10.5 kWh (1 kWh = 1,000 Wh)
• Total cost: 10.5 kWh × $0.24/kWh =$2.52

Why C is correct:

• Matches formula for electrical cost: power (kW) × hours × rate = 0.1 kW × 105 h × $0.24 =$2.52

Why the others are wrong:

• A: Underestimates by using only 1.5 hours total (ignores full time period)
• B: Calculates for one week only (5 h/day × 7 days × 0.1 kW × $0.24)
• D: Minor miscalculation of days or hours (e.g., 20 days instead of 21)

Final answer: C