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13 October, 14:49

The monthly (30 days) electric bill included the cost of running a central air-conditioning unit for 2.0 hr/day at 5000 W, and a series connection of ten 60 W light bulbs for 5.0 hr/day. How much did these items contribute to the cost of the monthly electric bill if electricity costs 8.0¢ per kWh?

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  1. 13 October, 15:06
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    Cost of air-conditioning unit = $ 24

    Cost of ten light bulbs = $ 7.2

    Explanation:

    For air-conditioning unit:

    N = number of days = 30

    t = time of usage per day = 2 hr

    T = Total time for which air-conditioning unit is used = N t = (30) (2) = 60 hr

    P = Power of unit = 5000 W

    c = electricity cost = 8.0¢ per kWh

    C = Total cost

    E = Total energy used

    Total energy used is given as

    E = P T

    E = (5000) (60)

    E = 300,000 Wh

    E = 300 kWh

    Total Cost for air-conditioning unit is given as

    C = E c

    C = (300) (8)

    C = 2400 ¢

    C = $ 24

    For the Bulbs:

    N = number of days = 30

    t = time of usage per day = 5 hr

    n = number of bulbs = 10

    T = Total time for which air-conditioning unit is used = N t = (30) (5) = 150 hr

    P = Power of each bulb = 60 W

    c = electricity cost = 8.0¢ per kWh

    C = Total cost

    E = Total energy used for bulbs

    Total energy used is given as

    E = n P T

    E = (10) (60) (150)

    E = 90,000 Wh

    E = 90 kWh

    Total Cost for bulbs is given as

    C = E c

    C = (90) (8)

    C = 720 ¢

    C = $ 7.2
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