Question

calculate the cost of operating heater of 500wfor 20 hrs at the rate of 3.90/unit

Answer 1

Hey bro here is the answer

Power = 500 W  i.e = .5 kW 

Time   = 20 h

rate     = Rs. 3.90 / unit

According to the formula:

Energy ( in kWh ) = Power ( in kW ) x Time ( in h)

Substituting: 

Energy ( in kWh ) = .5 kW  x  20

Therefore:

Energy = 10 kWh

Using unitary method:

      Energy       |      Rate / unit

______________________________

    1 kWh           |         Rs. 3.90 / unit

so

   10 kWh         |         Rs. x

x = 10 x 3.90

So the cost for operating a heater of 500 W for 20 h at the rate of 3.90 / unit is Rs. 39