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Homework Help: Time in min. it takes Jessica to bicycle to school

  1. Dec 6, 2008 #1
    1. The problem statement, all variables and given/known data

    The time in minutes (min.) it takes Jessica to bicycle to school is normally distributed with mean 15 and variance 4. Jessica has to be at school at 8:00 am. What time shoud she leave her house so she will be late only 4% of the time?
    A. 8:00 B) 11.5 min. before 8:00 C) 22 min. before 8:00 D) 18.5 min. before 8:00.
    Answer: D) 18.5 min. before 8:00.

    2. Relevant equations

    Z = 1.96 for 95% CI
    phat = 0.04; qhat = 0.96
    +/- Z[phat*qhat/sqrt(n)]
    15 +/- Z[phat*qhat/sqrt(n)]

    3. The attempt at a solution
    15 +/- Z[phat*qhat/sqrt(n)]
    15 +/- 1.96[0.04*0.96/sqrt(60)]
    = 0.025 min <--incorrect

    The answer should be D) 18.5 min. But where did i go wrong? How do i solve this problem? Thanks.
  2. jcsd
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