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Average Speed

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kayser Just gettin' started! Default Avatar
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Average Speed Post Thu Feb 05, 2009 12:46 am
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  • Lap #[LAPCOUNT] ([LAPTIME])
    On a recent trip, Mary drove 50 miles. What was the average speed at which she drove the 50 miles?

    (1) She drove 30 miles at an average speed of 60 miles per hour and then drove the remaining 20 miles at an average speed of 50 miles per hour.

    (2) She drove a total of 54 minutes.



    How would you do this problem using statement 1, if it were a Problem Solving question?

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    DanaJ Site Admin
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    Post Thu Feb 05, 2009 2:01 am
    Each statement by itself is sufficient.

    Take 1: Mary drove for 30/60 = half an hour with an average speed of 60 miles/h and then she drove for 20/50 = 0.4 hours with an average speed of 50 miles/h. This means she drove 50 miles in 0.9 hours, which gives us an average speed of 50/0.9 = 55.56 miles/h

    Take 2: Total distance = 50 and total time = 54 mins = 0.9 h. So average speed = total dist/total time = 50/0.9 = 55.56 miles/h.

    Answer D.

    kayser Just gettin' started! Default Avatar
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    Post Fri Feb 06, 2009 11:04 am
    So, for the first statement - if this was a PS question - is there a shortcut?

    Something like the weighted averages, or speed of a boat upriver & downriver... where the distance of each part of the trip is the same. Is there a similar shortcut here that I use instead of finding Total time and Total distance.

    Thanks alot.

    DanaJ Site Admin
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    Post Fri Feb 06, 2009 11:45 am
    Well, trust me, it's pretty short if you just write down the equations. I don't really know of any shorter method, but maybe someone else can help.

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    Stuart Kovinsky GMAT Instructor
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    Post Fri Feb 06, 2009 12:38 pm
    kayser wrote:
    So, for the first statement - if this was a PS question - is there a shortcut?

    Something like the weighted averages, or speed of a boat upriver & downriver... where the distance of each part of the trip is the same. Is there a similar shortcut here that I use instead of finding Total time and Total distance.

    Thanks alot.
    A great way to attack multiple-part speed questions is with a chart.

    Across the top of the chart, write the formula, D = r * t.
    Down the left side of the chart, write the different parts of the journey, e.g.:

    part 1
    part 2
    total

    or

    uphill
    downhill
    total

    So, for this question, your chart will look something like this (excuse my poor formatting!):

    .................Distance....=...........Rate.........*...........Time

    Part 1...........30........................60...............................

    Part 2...........20........................50...............................

    Total.............50........................x................................

    The chart allows you to quickly assimilate the information you're given and plan your route to solve the question.

    In this case, we'd see that we can calculate the times for the individual legs, then if we add those together we get total time. Since:

    Avg Rate = Total Distance/Total Time,

    we'd then have everything we need to calculate the final answer.

    The chart is simply a visualization of the equations to which Dana referred, but a lot of people love it because it lays out all the pieces so neatly.

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