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Math Help - Problem of direct variation

  1. #1
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    Problem of direct variation

    A car is traveling at speed s (in mph) on a dry asphalt road, and the brakes are suddenly applied. The stopping distance d (in feet) varies directly as the square of the speed s. If a car traveling at 60 mph can stop in 120 feet, what is the stopping distance of a car traveling at 70 mph?

    I believe the correct answer is 140 feet. Is this right?
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by mt_lapin View Post
    A car is traveling at speed s (in mph) on a dry asphalt road, and the brakes are suddenly applied. The stopping distance d (in feet) varies directly as the square of the speed s. If a car traveling at 60 mph can stop in 120 feet, what is the stopping distance of a car traveling at 70 mph?

    I believe the correct answer is 140 feet. Is this right?
    no. we have d = ks^2 for some constant k

    we are told, when d = 120,~s = 60. use this fact to solve for k.

    then plug in the value for k into the original equation. can you now find d if s = 70?
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