Question

There are 5 tube-lights each of 40 W in a house. These are used on an average for 5 hrs/day. In addition, there is an immersion heater of 1500W used on an average for 1hr/day. The number of units of electricity that are consumed in a month of 30 days is:
(A) 25 units
(B) 50 units
(C) 75 units
(D)100 units

Answer 1

To answer this question we should be knowing the equation to find the units of electricity that are consumed. Then we have to put the values that are present in the question. On solving the main equation, we will obtain the required answer.

Complete step by step answer:
We should know that the formula to find the number of units of electricity that are consumed in a month of 30 days is given by:
$E = {n_1}{P_1}{t_1} + {n_2}{P_2}{t_2}$
Here E denotes the electricity that is consumed.
${n_1},{n_2}$ are the number of hours the 5 tube-lights and the heater are used in a day respectively.
${P_1},{P_2}$ are the power of the 5 tube-lights and the heater respectively.
${t_1},{t_2}$ are the number of days the 5 tube-lights and the heater are used.
So we have to put the values in the formula from the question:
$E = 5 \times 40 \times 5 \times 30 + 1 \times 1500 \times 30 \\\ \Rightarrow E = 200 \times 150 + 45000 \\\ \Rightarrow E = 75 \\\$
Since E denotes the amount of power that is consumed so E = 75 kWh.
So we can say that the number of units of electricity that are consumed in a month of 30 days is75 units.

Hence the correct answer is option C.

Note: To avoid any confusion while solving such questions we should be knowing the basic idea behind the solution. It is known to us that the unit of electrical energy is kilowatt-hour or we can also say kWh. This is found when we multiply the power use, expressed in kilowatts or Kw by the number of hours of operation. This gives us the power that is consumed. Then on multiplying the value with the cost per kWh we can obtain the cost of the total amount of energy.