Question 92104
You are told that the initial population of the town in 2010 is 1000. Each year that passes
the town adds another 1000 persons.
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So the population is projected to be:
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In 2010 ... t=o ... Population = 1000 <=== this is Po
In 2011 ... t=1 ... Population = 2000
In 2012 ... t=2 ... Population = 3000
In 2013 ... t=3 ... Population = 4000
In 2014 ... t=4 ... Population = 5000
In 2015 ... t=5 ... Population = 6000

and so on ...
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The rate of change is 1000 per year. This will be the slope of the graph because each year
that passes results in an increase of 1000 in the population.
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Using the equation form P=mt+b, we can replace m with 1000 to get:
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{{{P = 1000t + b}}}
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in which t is the year of interest minus 2010. 
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Suppose our year of interest is 2010. Then t = 2010 - 2010 = 0
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We know that in 2010 the population P is 1000. So we can substitute 1000 for P and 1000
for m to make the equation become:
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{{{1000 = 1000*0 + b}}}
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Do the multiplication on the right side to get:
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{{{1000 = 0 + b}}}
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This tells us that b is 1000. Substitute this value for b in the equation, and it becomes:
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{{{P = 1000t + 1000}}}
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Let's check this out. Suppose the year we are interested in finding the population 
is 2015 ... 5 years after 2010 so that t = 5. Substitute t = 5 and the equation becomes:
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{{{P = 1000*5 + 1000 = 5000 + 1000 = 6000}}}
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That is exactly as our table above said it would be.
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So all you have to do is pick a year for wish you want to find the population.  Subtract
2010 from that year. Use the result of that subtraction as the value of t in the equation:
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{{{P = 1000*t + 1000}}}
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For example.  Suppose the year you are interested in finding the population for is 2030.
Subtract 2010 from 2030 and get 20 for the value of t. Substituting 20 for t results in
the equation becoming:
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{{{P = 1000*20 + 1000}}}
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The multiplication on the right side makes the equation become:
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{{{P = 20000 + 1000 = 21000}}}
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In 2030 the population will be 21,000.
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Hope this helps you to understand the problem a little better.