Identify the slope, m. This can be done by calculating the slope between two known points of the line using the slope formula.
Find the y-intercept. This can be done by substituting the slope and the coordinates of a point (x, y) on the line in the slope-intercept formula and then solve for b.
A suitable probability app shows the 1st percentile to be at about 11040 pages.
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The app shown has an error in the first decimal digit. The number is closer to 11040.82301. We expect the manufacturer would round to the nearest 10 or 50 or 100 in advertising.
Rearrange the equation so it looks like y=mx+b
y=

+5
m is the slope
m=
The box will cost $160 once the coupon is used (not includeing tax)