Question

A washing machine rated $300W$ is operated for one hour/day. If the cost of a unit is Rs $3.00$ then the cost of the energy to operate a washing machine for the month of March is

• A. $Rs25.60$
• B. $Rs27.50$
• C. $Rs27.90$
• D. $Rs26.90$

Answer 1

The correct option is C $Rs27.90$

In this case, the washing machine that is rated $300W$ is operated for $1$ hr/day. The electric bill has to be calculated for the month of March. That is, the total number of hours the washing machine is operated in this month is $31$ hours as this month has $31$ days.
The total energy that is used by the washing machine in a day is given by the formula $Q=Pt$.
Here, the power is $300W$ and the time used is $1hour$.
So, $Q=300W\times 1h=300Wh=0.3kWh$
Therefore, the total number of $kWh$ for a month is $0.3kWh\times 31=9.3kWh$
The cost of $1kWh$ is given as $Rs.3$.

Hence, the total cost for a month is $=9.3kWh\times 3=Rs.27.90$