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Weight Chart finding average

  1. Jan 14, 2012 #1
    1. The problem statement, all variables and given/known data

    The weight of a baby measured over an 11-month period is given in the weight chart below for this problem.
    Weight (lb) Age (months)
    6.6 0 (birth)
    7.4 1.0
    9.6 2.0
    11.2 3.0
    12.0 4.0
    13.6 5.0
    13.8 6.0
    14.8 7.0
    15.0 8.0
    16.6 9.0
    17.5 10.0
    18.4 11.0

    (a) What was the average monthly weight gain for this baby over the period from birth to 3 months?
    ------- lb/month

    (b) What was the average monthly weight gain for the baby over the period from 8 months to 11 months?
    --------lb/month

    (c) If a baby continued to grow at the same rate as in the first 3 months of life, what would the child weigh at age 12 years?



    For part A I added the weights and divided by 3 but I got the wrong answer..... so how do I do it?

    Im not sure how to solve parts B and C


    please help
     
  2. jcsd
  3. Jan 15, 2012 #2

    cepheid

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    You should read the question more carefully. What you have computed is the average weight. What the question is asking for is the average weight GAIN over the course of a month.

    Part b is the same as part A, only the average is being taken over a different time interval.

    For part C, think about how you'd compute the total growth, if you've been given a growth RATE.
     
  4. Jan 15, 2012 #3
    So for part A I should subtract 7.4-6.6
    9.6-7.4
    11.2 -9.6

    add the totals together and divide by three?
     
  5. Jan 15, 2012 #4
    Ok thanks I got A and B.
    For part C Im not sure, Do I divide 1.53 (from part A) by 12?
     
  6. Jan 15, 2012 #5

    cepheid

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    Yeah

    You have a RATE of weight gain in pounds/month from part A. Let's say for example, the growth rate is 3 lbs and the starting weight is 5 lbs. If the growth rate remains constant, how much will the baby weight after one month? After two months? Now, how did you compute that? Hint: look at the units? what do you have to multiply a rate in pounds/month by in order to get the total weight gain, which is in pounds?
     
  7. Jan 16, 2012 #6
    Im not really understanding what your trying to explain.
    so the rate of weight is not used?
     
  8. Jan 16, 2012 #7

    cepheid

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    Just answer this question here:

    I'm sure you know how to answer that. Once you do, ask yourself, "how did I do that?" It's same as the method for part C.

    It's the same method in general for whenever you're given a *rate of change* of some quantity with time, and you want to figure out the total change in that quantity over some time interval. Another example: say I'm moving along in my car at 50 km/h. After 3 hours, how far will I have moved? You find the answer in exactly the same way, only this time the quantity in question is distance rather than weight.
     
  9. Jan 16, 2012 #8
    okkk now I understand what you mean thank you!!!!!
     
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