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Homework Help: Two growth equations

  1. Sep 4, 2006 #1
    In 1980, there were about 170 million vehicles (cars and trucks) and about 227 million people in the United States. The number of vehicles has been growing at 4% a year, while the population has been growing at 1% a year. When was there, on average, one vehicle per person?

    hmm, I think since we're looking for there to be one vehicle per person, then the number of people and the number of vehicles need to be equal. I'll set the two equations equal to each other.

    P (in millions)
    P = 170(1.04)t <----- for vehicles
    P = 227(1.01)t <----- for people

    Setting them equal to each other because we're looking for one vehicle per person....
    170(1.04)t = 227(1.01)t
    ln[170(1.04)t] = ln[227(1.01)t]
    ln170 + ln1.04t = ln227 + ln1.01t
    ln170 + tln1.04 = ln227 + tln1.01
    tln1.04 - tln1.01 = ln227 - ln170
    t(ln1.04 - ln1.01) = ln227 - ln170
    t = (ln227 - ln170) / (ln1.04 - ln1.01)
    t = 9.87864 years

    1980 + 9.87864 = 1989.88

    Is this correct?
  2. jcsd
  3. Sep 4, 2006 #2


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    Staff Emeritus
    Science Advisor

    Certainly looks correct. One could put the answer back into the equations to see if the curves defined by equations intersect, the P's equal at the same time.
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