Given
Power = 40W
Time = 8 hours
Now
Power = \(\frac{Energy}{Time}\)
⇒ 40 = \(\frac{Energy}{8}\)
⇒Energy = 40 x 8 = 320
Energy consumed by 2 bulbs in 1 week = 320×2×7 = 4480 Wh = 4.48 kW
(∵ 1kWh = 1000Wh)
Total units = 4.48 units
Now
Cost of 1 unit = Rs 3
Total cost = 4.48 ×3 = Rs13.44
Total cost for 1 week is Rs 13.44