Question

A house is fitted with 5 tubes each of 40 W. if all the tubes are lighted for 20 hours and if cost of electrical unit is Rs.0.50, the total cost of electricity consumed is:
A. Rs.20
B. Rs.4
C. Rs.5
D. Rs.2

Answer 1

Hint: We know that 1 unit is equivalent to 1000 W or 1 KW, so to solve this problem we need to calculate total power consumed by 5 tubes and need to convert in the form of kilowatt, so we can easily calculate total number of units consumed in 20 hours and by multiplying by cost per unit we can calculate our required answer.

Complete Step by step solution:
First we need to calculate total electrical power consumed by 5 tubes which can be calculated as,
Total power=number of tubes x power of single tube,
Total power= $5\times 40=200W$
Now, we know that 1 Kilowatt = 1000W so, 200 W will be equivalent to-
$=\dfrac{200W}{1000W}=0.2$ Kilowatt,
We also know that 1 unit is equal to 1 kilowatt per hour, so the total number of units from 0.2 Kilowatt will be 0.2 units per hour.
Now, the total number of units in 20 hours will be, $0.2\times 20=4$units.
Given per unit cost = 0.50 per unit so,
Total cost of electrical unit = $0.50\times 4$= 2Rs .
Hence, option D is correct.

Additional information:
The above mentioned method to calculate total cost of electrical unit is widely used for home as well as commercial uses, but the only difference is that we don’t need to consider each and every appliances or machine to calculate power, instead of that regular reading from the uses of these appliances has been updated by electric meter and finally the cost is derived from the total number of Kilowatt power used in the cycle.

Note:
While calculating total number of units carefully convert the watt to Kilowatt and Kilowatt per hour accordingly to avoid calculation mistakes and after that total cost of electric power consumed can be calculated