Question 894195
 In deep sea diving, the pressure exerted by water plays a great role in designing underwater equipment.
 If at a depth of 10 feet there is a pressure of 21 lb/in (squared), and at a depth of 50 feet there is a pressure of 75 lb/in (squared), write an equation giving a relationship between depth and pressure.
 Use this relationship to predict pressure at a depth of 100 feet.
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I don't get that either, my calculations:
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The slope formula; m = {{{(y2-y1)/(x2-x1)}}}
Assign the given values as follows:
x1=10; y1=21
x2=50; y2=75
Find the slope
m = {{{(75-21)/(50-10)}}} = {{{54/40}}} = {{{27/20}}} is the slope
now we have
{{{27/20}}}x + b = y
go to decimals
1.35x + b = y
Replace x with 10 and y with 21, find b
1.35(10) + b = 21
13.5 + b = 21
b = 21 - 13.5
b = 7.5
the equation
y = 1.35x + 7.5
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Use this relationship to predict pressure at a depth of 100 feet.
y = 1.35(100) + 7.5
y = 135 + 7.5
y = 142.5 psi at 100 ft
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I put this equation in my Ti83, looked at the table and it agreed with the given x/y values. 
The equation they give does not do that. For x=10, it comes up with y=28.7!
Book is wrong!