How to answer these math question

The annual cost of owning a car varies partially as the distance driven. The cost is $3700 for 10 000 km, and $7450 for 25 000km.

1)What is the fixed cost of owning a car?
2)Determine the equation relating the total cost to the distance driven
3)Find the annual cost if 15000 km are driven.

Please and thank you!

We calculate cost per km (variable cost):
($7450 - $3700) / (25000km - 10000km)
= $3750 / 15000 km
= $0.25 / km

So when driving 10000 km, the variable cost is
$0.25/km * 10000 km = $2500

Total cost for 10000 km is $3700

Fixed cost = $3700 - $2500 = $1200

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Let C = total cost
Let d = distance driven

C = 1200 + 0.25 d

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Check:

d = 10000km -----> C = 1200 + 0.25(10000) = 1200 + 2500 = $3700
d = 25000km -----> C = 1200 + 0.25(25000) = 1200 + 6250 = $7450

ok

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Annual cost if 15000 km are driven

C = 1200 + 0.25(15000) = 1200 + 3750 = $4950

Let the annual cost of owning a car be C, and the distance driven be D.

It is given that the annual cost varies partially as the distance driven.
This means that the annual cost consists of two component - a fixed component and a varying component that varies as per D.
So, we can write the expression for C as:

C = kD + m .........(1)

Here m is the fixed cost of owning the car, and k is a constant introduced because C is directly proportional to D.

Now, the cost is $3700 for 10,000 km and $7450 for 25,000 km.
Thus we get two equations in two variables:

3700 = 10000k + m ........(2)
7450 = 25000k + m ........(3)

Subtracting (3) from (2), we have:

-3750 = -15000k
=> k = 0.25

Substituting this value of k in eqn. (2), we have

3700 = 10000*(0.25) + m
=> 3700 = 2500 + m