The population of Laredo, Texas, was about 215,500 in 2007. It was about 123,000 in 1990.

Question

If we assume that the population growth is constant and y represents the number of years after 1990, write a linear equation to find p, Laredo's population for any year after 1990.

Answer 1

Hi Stacy.  This can be solved using your standard rise over run slope calculation.  Think of the number of years after 1990 as your input variable.  2007 would be 17 years after 1990, so the "y" variable would be 17.  This is your "run", and it goes in the denominator of your slope calculation.  Then the difference between the 2007 population and the 1990 population would be the "p" variable difference, the "rise", which is 92500.  Dividing, you will get 5441.17.  Since we're talking about humans, we can't have a decimal, so let's round up to 5442.

 

Using the slope-intercept form of a linear equation (y = ax + b), we will say that p = 5442y + 123000.  Do you know why we add the 123000?  It's because that's the starting value, which is equivalent to the "intercept" in the slope-intercept form.

 

Hope this helps.

 

Angie