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    ecentric191's Avatar
    ecentric191 Posts: 4, Reputation: 1
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    #1

    Apr 3, 2011, 02:35 PM
    On a trip students drove 120 miles at 40 miles per hour and another 120 miles at 60
    Miles per hour. What was their average speed for the trip?
    JudyKayTee's Avatar
    JudyKayTee Posts: 46,503, Reputation: 4600
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    #2

    Apr 3, 2011, 02:37 PM

    We don't do homework as such. We don't provide answers.

    Work this out, post what you believe, someone will come along to look at it.
    ecentric191's Avatar
    ecentric191 Posts: 4, Reputation: 1
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    #3

    Apr 3, 2011, 02:38 PM
    Well I do not know how to do this problem... so that's why I posted it because I need help on it.
    ecentric191's Avatar
    ecentric191 Posts: 4, Reputation: 1
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    #4

    Apr 3, 2011, 02:38 PM
    Comment on JudyKayTee's post
    Well I do not know how to do this problem... so that's why I posted it because I need help on it.
    ballengerb1's Avatar
    ballengerb1 Posts: 27,378, Reputation: 2280
    Home Repair & Remodeling Expert
    		  
    #5

    Apr 3, 2011, 02:41 PM

    One person's speed is 40 mph and anothers is 60 mph. Are you saying you can't figure the average speed betweem 40 and 60?? The 120 miles has nothing to do with the problem.
    JudyKayTee's Avatar
    JudyKayTee Posts: 46,503, Reputation: 4600
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    #6

    Apr 3, 2011, 02:42 PM

    Well, we don't do homework. I did mine; it's your turn to do yours.

    Here's a hint - figure out miles per hour for first leg, miles per hour for second, divide by two to get average.

    What grade are you in?
    eawoodall's Avatar
    eawoodall Posts: 230, Reputation: 5
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    #7

    Apr 3, 2011, 05:34 PM
    you must remember your formula

    rate = distance divided by time.
    r =d/t.

    similarly multiplying boths sides by t/r gives:
    t = d/r.
    plug in the numbers:
    t = 120 miles/ (40 miles/1 hour) + 120 miles/ (60 miles/1 hour)
    t = 120 miles (1 hour/40 miles) + 120 miles (1 hour/60 miles)
    t = 120 mile hours/40 miles +120 mile hours /60 miles
    cancel like terms in both numerator and denominator
    t = 120 hours/40 + 120 hours /60
    t = 3 hours + 2 hours
    t = 5 hours

    the trip took 5 total hours.
    120 miles + 120 miles = 240 miles
    r = d/t
    r = 240 miles /5 hours
    r = 48 miles per hour.

    as long as you remember you basic formula you can manipulate it.
    r =d/t.
    multiply by t, and we get
    d = rt.
    this is a different form of the same equation r = d/t.
    both tell you the same thing. You choose which is easier to solve.
    ecentric191's Avatar
    ecentric191 Posts: 4, Reputation: 1
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    #8

    Apr 10, 2011, 04:26 PM
    Comment on JudyKayTee's post
    I am in 8th grade but my teacher does not explain anything she says some things about them and then tells us our homework and barely anyone knows how to do it...
    JudyKayTee's Avatar
    JudyKayTee Posts: 46,503, Reputation: 4600
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    #9

    Apr 10, 2011, 04:38 PM

    Then you and your classmates or you and your parents need to speak with the teacher. If no one in the class can figure out the problems because she explains nothing something should be done to remedy the situation.
    Communist's Avatar
    Communist Posts: 16, Reputation: 3
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    #10

    Apr 11, 2011, 12:38 AM
    The 120 miles is only mentioned to show that the two distances are the same. i.e. they drove the same distance at 40mph as they did at 60mph.

    This means that you can find the average of the two numbers (40 and 60) to find the average speed.
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #11

    Apr 11, 2011, 08:22 AM
    Quote Originally Posted by Communist View Post
    The 120 miles is only mentioned to show that the two distances are the same. i.e. they drove the same distance at 40mph as they did at 60mph.

    This means that you can find the average of the two numbers (40 and 60) to find the average speed.
    NO! That's not correct.

    I'm afraid that this is not the correct way out of it. Computing using the long way (ie finding the total distance and the total time and working the average speed through those) does not give the same answer, meaning that simply averaging the speed gives a wrong answer. You might take longer when using the basics, but you are making sure you are not making any false assumptions.
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