Question 762774
If the record company charges you $12 for five cds, and $20 for nine cds. How much would you pay for fifty cds. 
<pre>
We must get the equation.

Let y = the total cost and x = the number of cds.

So when x = 5, y = $12 and
   when x = 9, y = $20

So the problem becomes:

Find the equation of the line through the points (5,12) and (9,20)

Slope formula:

m = {{{(y[2]-y[1])/(x[2]-x[1])}}}

where (x<sub>1</sub>,y<sub>1</sub>) = (5,12)

and where (x<sub>2</sub>,y<sub>2</sub>) = (9,20)

m = {{{(20-12)/(9-5)}}}

m = {{{8/4}}}

m = 2

We use the point-slope formula:

y - y<sub>1</sub> = m(x - x<sub>1</sub>)

where (x<sub>1</sub>,y<sub>1</sub>) = (5,12) and m = 2

y - 12 = 2(x - 5)

y - 12 = 2x - 10

     y = 2x + 2

To answer:
</pre>
How much would you pay for fifty cds? 
<pre>

Substitute x = 50

     y = 2x + 2

     y = 2(50) + 2    

     y = 100 + 2

     y = 102

Answer $102.

Edwin</pre>