Results 1 to 4 of 4

Math Help - Proportionality

  1. #1
    Member
    Joined
    Sep 2009
    Posts
    145

    Proportionality

    Hell-o,

    I'm primarily looking for alternative solutions, assuming my method wasn't a fluke

    The stopping distance of a car is the number of feet that the car travels after the driver starts applying the brakes. The stopping distance of a certain car is directly proportional to the square of the speed of the car, in miles per hour, at the time the brakes are first applied. If the carís stopping distance for an initial speed of 20 miles per hour is 17 feet, what is its stopping distance for an initial speed of 40 miles per hour?

    \displaystyle \frac{20}{17} = \frac{40}{x}

    \displaystyle x = \frac{680}{20}

    \mathrm{x = 34\ feet}

    This answer is incorrect, but I realized that multiplying it by 2 gives the correct answer (68). Is this method a fluke?
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor
    Joined
    Mar 2010
    From
    Florida
    Posts
    3,093
    Thanks
    5
    40=2*20

    17*2=x

    It will work if it is easy to see or an integer.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Newbie
    Joined
    Jan 2011
    From
    NSW, Australia
    Posts
    15
    To quote: "The stopping distance of a car is directly proportional to the square of the speed of the car."

    So the structure for your working should actually be:

    \displaystyle \frac{20^2}{17} = \frac{40^2}{x}

    Which is the same as...

    \displaystyle \frac{400}{17} = \frac{1600}{x}

    When you attempted the solution, you used a number on your second fraction which was double the original amount, when in actual fact it should have 4 times. That's why you needed to multiply by 2 again to achieve the correct answer. Your method is fine, otherwise.

    Hope that helps. :V
    Follow Math Help Forum on Facebook and Google+

  4. #4
    MHF Contributor
    Joined
    Dec 2009
    Posts
    3,120
    Thanks
    1
    Quote Originally Posted by Hellbent View Post
    Hell-o,

    I'm primarily looking for alternative solutions, assuming my method wasn't a fluke

    The stopping distance of a car is the number of feet that the car travels after the driver starts applying the brakes. The stopping distance of a certain car is directly proportional to the square of the speed of the car, in miles per hour, at the time the brakes are first applied. If the carís stopping distance for an initial speed of 20 miles per hour is 17 feet, what is its stopping distance for an initial speed of 40 miles per hour?

    \displaystyle \frac{20}{17} = \frac{40}{x}

    \displaystyle x = \frac{680}{20}

    \mathrm{x = 34\ feet}

    This answer is incorrect, but I realized that multiplying it by 2 gives the correct answer (68). Is this method a fluke?
    You've incorrectly set up your proportions.

    The proportion involves the "square" of the car's speed.

    \displaystyle\frac{20^2}{17}=\frac{40^2}{x}\Righta  rrow\frac{20^2}{17}=\frac{2^2\;20^2}{x}

    \displaystyle\ x=\frac{20^2\;2^2}{20^2}17=2^2\;(17)
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Proportionality
    Posted in the Algebra Forum
    Replies: 1
    Last Post: March 12th 2011, 10:51 AM
  2. (SAT) Proportionality
    Posted in the Algebra Forum
    Replies: 3
    Last Post: January 11th 2011, 05:43 AM
  3. inverse proportionality
    Posted in the Math Topics Forum
    Replies: 10
    Last Post: January 26th 2010, 09:48 AM
  4. inverse proportionality
    Posted in the Math Topics Forum
    Replies: 1
    Last Post: January 19th 2010, 08:22 AM
  5. Proportionality
    Posted in the Algebra Forum
    Replies: 7
    Last Post: October 22nd 2008, 01:11 PM

Search Tags


/mathhelpforum @mathhelpforum