Question

An electric lamp is marked 60 W, 230 V. The cost of a 1 kWh of energy is Rs1.25. The cost of using lamp 8 hrs a day for 30 day is
(A) Rs 10
(B) Rs 16
(C) Rs 18
(D) Rs 20

Answer 1

The wattage and the voltage that are marked in the electric lamp tell us about the power consumption of the electric lamp. Knowing the rate of electric rate charges (which is Rs1.25) and time the bulb is used by the person, we can calculate the net cost of using the lamp for the whole month.

Complete answer:
The electric lamp is marked as 60 W, 230 V (it consumes 60 watts at a given constant voltage of 230 volts)
We know that the wattage of a lamp is given as 60 watts
Now we know that the cost of 1kWh of electric energy is Rs 1.25.
Total time in hours for which the electric lamp is used is for 8 hrs a day for 30 days.
Total wattage consumed by the electric lamp $= 60 \times 8 \times 30 = 14400W$
Total wattage consumed in kWh by the electric lamp $= \dfrac{{60 \times 8 \times 30}}{{1000}} = 14.4kWh$
Total cost for the usage of the electric lamp $= 14.4 \times 1.25 = 18$
The total cost for the usage of the electric lamp is Rs 18 the correct option is (C).

Note:
The voltage that is marked on the lamp which here is 230 Volts is just given without having any kind of use in this question. Many times, students make mistakes rather than getting confused on deciding where the value of the voltage given in the question should be used or not. This takes out quite a time as the wattage of the lamp is given directly to us. We only use the value of wattage to find the price of the electric charge.