You and your housemate have an argument over the cost of the electric bill. You want to turn off the outside porch light before going to bed each night. Your housemate wishes to keep the light on from 6:00 PM each evening to 6:00 AM the next morning and says that this will not add any significant amount to the electric bill. If the porch light bulb is a 65 W bulb, how much will your housemate's plan contribute to the electric bill each month? Take the number of days in a month as 30 and the average cost of electricity is 17.0¢/kW · h.

  Cost = $ 3,995

Explanation:

Let's start by looking for the energy consumed, which is the power over time

               E = P t

The power is 65 W, the time is 12 h each day, in a period of 30 days the time is

            t = 12  30 = 360 h

            E = 65 360

            E = 23,500 W h

We reduce to KW h

            1 kW = 1000 W

            E = 23.5 kW h

Let's use a direct ratio rule (rule of three) to find the cost

           Cost = 23.5  17.0

           Cost = 399.5 c

           Cost = $ 3,995

a and b sometimes have a positive value