Question 109178
A computer store sells 10 floppy diskettes for $15, and 30 for $40. Assume that the number of dollars varies linearly with the number of diskettes.
:
a) Write the particular equation expressing dollars in terms of diskettes.
:
Let x = no. of floppies sold
Let y = cost of the item
:
find the slope of the equation, slope formula:  m = {{{(y2-y1)/(x2-x1)}}}
Using the given data assign as follows:
x1=10, y1=15; x2=30, y2= 40
:
m = {{{(40-15)/(30-10)}}} = {{{25/10}}} = 1.25 is the slope
:
Find equation using the point/slope formula: y - y1 = m(x - x1)
:
y - 15 - 1.25(x - 10)
y - 15 = 1.25x - 12.5
y = 1.25x - 12.5 + 15
y = 1.25x + 2.5; is the equation expressing cost in terms of no. of items
:
:
b) Predict the price for a box of 100.
:
Substitute 100 for x in the equation we just created:
y = 1.25(100) + 2.5
y = 125 + 2.5
y = $127.5 is the cost of 100 items
:
:
c) State the domain for your variable.
:
The domain would be all real integers => 1. You can't include 0 because in our
equation the cost of 0 items is $2.50, hardly reasonable.
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d) Sketch and label the graph.
You can plot the points given and draw a straight line thru them, it should look like this: 
y (vertical is cost) and x (horizontal) is no. of items sold
:
{{{ graph( 300, 200, -10, 110, -10, 140, 1.25x+2.5) }}}
:
How about this, you got a handle this now?