A family pays ₹ 1000 for electricity on an average in those months in which it does not use a water heater. In the months in which it uses a water heater, the average electricity bill is ₹ 1240. The cost of using the water heater is ₹ 8.00 per hour. Find the average number of hours the water heater is used in a day.

Step 1 : Formulation :

An assumption is that the difference in the bill is only because of using the water heater.

Let the average number of hours for which the water heater is used in a day be x.

Difference in bill per month due to using water heater = ₹ 1240 - ₹ 1000 = ₹ 240.

Cost of using water heater for 1 hour = ₹ 8.

So, the cost of using the water heater for 30 days = 8 × 30 × x.

Also, the cost of using the water heater for 30 days = Difference in bill due to using water heater.

So, 240 x = 240 …..(1)

Step 2 : Finding the solution :

By solving equation (1), we get :

⇒ x = $\dfrac{240}{240}$ = 1.

Step 3 : Interpretation :

Since, x = 1

The water heater is used for an average of 1 hour in a day.