Question

Your utility company charges 13 cents per kilowatt-hour of electricity. What is the daily cost of keeping lit a 75 watt light bulb for 12 hours each day? How much will you save in a year if you replace the bulb with an led bulb that provides the same amount of light using only 15 watts of power?

Answer 1

.13×75=9.75 per hour
9.75×12=117 per 12 hours
117×365=42,705 per year

.13×15=1.95 per hour
1.95×12=23.4 per 12 hours
23.4×365=8,541 per year

so 42,705-8,541=34,164 saved by using an led instead

Answer 2

we know that

$1 Kw=1000 w$

The company charges $13$ cents per kilowatt-hour of electricity

$13 cents=0.13 dollars\\\\ 75w=\frac{75}{1000} kw\\\\ 15w=\frac{15}{1000} kw$

Step $1$

Find the cost for the light bulb

a) in one day

$\frac{75}{1000} *12=0.9\ kw\ hour$

$Cost=0.13*0.9$

Cost=$$0.117$

b) In a year

$\frac{75}{1000} *12=0.9\ kw\ hour$

$0.9*365=328.5\ kw\ hour$

$Cost=0.13*328.5$

Cost=$$42.71$

Step $2$

Find the cost for the led bulb

a) in one day

$\frac{15}{1000} *12=0.18\ kw\ hour$

$Cost=0.13*0.18$

Cost=$$0.0234$

b) In a year

$\frac{15}{1000} *12=0.18\ kw\ hour$

$0.18*365=65.7\ kw\ hour$

$Cost=0.13*65.7$

Cost=$$8.54$

Step $3$

Find the difference in cost

$Save=42.71-8.54\\ Save=34.17 dollars$

therefore

the answer is

$34.17 dollars$