Question 198309
The 1st step is to decide what goes on the x-axis
and what goes on the y-axis.
Q: Does miles driven determine how much is left
in the tank?
Does how much is left in the tank determine the
miles?
The 1st statement is true, so y-axis is the gallons
left in tank and x-axis is the miles driven
(x causes y)
I'm given 2 points, (x,y):
(100,12.8)
(300,6.4)
The formula is:
{{{(y - 6.4)/(x - 300) = (12.8 - 6.4)/(100 - 300)}}}
{{{(y - 6.4)/(x - 300) = 6.4/-200}}}
Multiply both sides by {{{-200*(x - 300)}}}
{{{(y - 6.4)*(-200) = 6.4*(x - 300)}}}
{{{-200y + 1280 = 6.4x - 1920}}}
{{{200y = -6.4x + 3200}}}
{{{y = -.032x + 16}}}
I checked the points (100,12.8) and (300,6.4) 
They are on this line
Here's a plot:
{{{ graph( 400, 400, -50, 550, -2, 20, -.032x + 16) }}}
If the driver drives 0 miles (uses no gas), there are
16 gallons in the tank (full)
{{{y = -.032*0 + 16}}}
{{{y = 16}}}