
According to the 2000 Census the population of Canton was 7,720. Holly Spring's population was 3,200. Canton's population increases at a rate of 80 people a year and Holly Spring's population increases by 120 people a year. Which equation may be used to determine the number of years before the populations are equal?
let
t = the number of years, counting from 2000, needed to have equal populations
C(t) = 7720 + 80t is the number of people in Canton t years after 2000
S(t) = 3200 + 120t is the number of people in Holly Springs t years after 2000
C(t) = S(t)
7720 + 80t = 3200 + 120t give the value for t counting from 2000 before the populations are equal
120t  80t = 7720  3200
40t = 4520
t = 4520/40
t = 113
since now we are 2013 and 2013  2000 = 13 there will be 113  13 = 100 years before the populations are equal.
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