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?

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