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To qualify for the race, each driver must complete 2 laps of the track at an average speed of 100 mi/h. Benjamin averages only 75 mi/h on his first lap. How fast must he go on his second lap to qulify for the race?

  • math -

    let the distance around the track be d km
    so the time needed to qualify is 2d/100

    his time for the first lap is d/75 hours
    let his speed for the second lap be r km/h
    so the time for his second lap is d/r hours

    total time = d/75 + d/r = (dr+75d)/75

    then (dr+75d)/75 = 2d/100

    divide both sides by d
    (r+75)/75 = 2/100

    you finish it, let me know what you got

  • math -

    i got 3675, but that doesn't make any sense, he has to go 3675 mph?

  • math -

    I believe Reiny meant to write
    d/75 + d/r = (dr + 75 d)/(75 r)
    =================================

    d/75 + d/r = 2d/100

    1/r = 2/100 - 1/75 = 6/300 - 4/300 = 1/150

    r = 150

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