Residents of the town of Maple Grove who are connected to the municipal water supply are billed a fixed amount yearly plus a charge for each cubic foot of water used. A household using 1000 cubic feet was billed $90, while one using 1600 cubic feet was billed $105.(adsbygoogle = window.adsbygoogle || []).push({});

a) What is the charge per cubic foot?

b) Write an equation for the total cost of a resident's water as a function of cubic feet of water used.

c) How many cubic feet of water used would lead to a bill of $130?

All right, here are my attempts. I first did the whole find the slope thing.

(1000, 90) , (1600, 105)

(105 - 90) / (1600 - 1000) = 15/600 = 1/40

So, this means that for $1, you get 40 cubic feet. (but that isn't the question... the question is how much does 1 cubic foot cost.....)

that was the first thing that confused me.. but i decided that maybe i was going in the right direction, so I tried to keep working... by trying to write the equation.

y = mx + b

90 = (1/40)(1000) + b

90 = 25 + b

b = 65

y = (1/40)x + 65

now that i had the equation, i plugged in a 1 for x to see what 1 cubic foot would cost.

y = (1/40) (1) + 65

y = 1/40 + 65

y = $65.025

so, now i subtracted that amount from the fixed amount

$65.025 - $65 = $.025 or 2.5 cents

What confuses me is ... .025 = 1/40 (my slope), but i thought the slope meant 1 dollar for 40 cubic feet, now 1 cubic foot for .025 cents. And if it does work out, i just would like to know why you can switch the variables around like that so easily.

so.. to continue..

i have the answers for a and b.. time to work out c.

130 = (1/40)x + 65

x = 2600

a) 2.5 cents/cubic ft

b)y = (1/40)x + 65

c)2600 cubic feet of water used

I think it's all correct, but i seriously want to know why the variables can be switched .. because the slope should be $1 for 40 cubic feet, and not 2.5 cents for 1 cubic foot. Yes, it does make sense, i just don't know why.. lol..

thanks in advance.

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