Kabir bought 5 new light bulbs of 50 W each in addition to the 7 bulbs he already had in his house which were also 50 W each. Calculate his electricity bill, if he keeps the new bulbs on for 5 hours and the older bulbs on only for 3 hours, and the cost of one unit of electricity is Rs. 60.

Explanation

New bulbs = 5, Old bulbs = 7: Total wattage of new bulbs = 5 × 50; Total wattage of old bulbs = 7 × 50: Time the new bulbs are kept on = 5 hours; Time the old bulbs are kept on = 3 hours The number of units of electricity consumed by new bulbs (n1) = (total wattage × time in hours) / 1000 = 50×5×5 / 1000 = 1.25 The number of units of electricity consumed by old bulbs (n2) = 50×7×3 / 1000 = 1.05 Total bill of electricity = number of units of electricity consumed × amount for one unit of electricity =(n1 - n2) × 60 = (1.25 - 1.05) × 60 = 138 Therefore, his electricity bill will be Rs. 138.