How can i find the answer?(Pre-calculus help!) ?

Question

One Freezer costs $723.95 and uses 90 kwh(kilowatt hours) of

electricity each month. A second freezer costs $600 and uses

100kwh each month. The expected life of each freezer is 12 years.

What is the minimum electric rate(in cents per kwh) for which the 12

year total cost( purchase cost + electrical cost) will be less for the

first freezer?

I believe that the answer is about 5 cents per kilwatt hour. I just cant

get the answer in using the correct formula. Please help.

Answer 1

First you need to assume the freezers are never unplugged, otherwise you will get a different answer.

Over 12 years, you have 144 months.

Assume electric rate = Y

Cost of first freezer = initial cost plus monthly cost = $723.95 + 90 kwh * 144 * Y

Cost of second freezer = initial cost plus monthly cost = $600 + 100 kwh * 144 * Y

These two costs are equal when 723.95 + 12960Y = 600 + 14400Y or 123.95 = 1440Y or Y = 8.61 cents, therefore this is the minimum cost where freezer 1 is less.

Answer 2

the answer is boobies

Answer 3

y - total cost first freezer after 12 years

z - total cost second freezer after 12 years

12 years = 144 months

x - cost per kilowatt

total cost = initial cost + rate of month * total month * price of kilowatt/hr in dollars.

y = 723.95 + 90 * total months * p/100 = 723.95 + 12960 * p/100

z = 600 + 100 * total month * p/100 = 600 + 14400 * p/100

When is min p if y < z.  Have to find out when y = z.

723.95 + 12960 * p/100 = 600 + 14400 * p/100

1440p/100 = 123.95

p = 8.61 => 9 cents.