PLEASE HELP! ADA practice exam QR 37, 38

chiba

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Apr 21, 2008
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    I searched this forum regarding these questions but was unable to find them.
    Could anyone PLEASE help me to solve these problems??

    #37: The numbers (1,2,3,6) have an average (arithmetic mean) of 3 and a variance of 3.5. What is the average (arithmetic mean) and variance of the set of numbers (3,6,9,18)
    answer is 9, 31.5

    How do you find variance?
    Isn't it
    S^2= sum (x-average)^2 / n-1 ?
    I don't know how to get 31.5!



    #38. Jill has 6 different books. in how many ways can Jill select 2 different books?
    answer is 30 but I got 15.
    let n=6
    let r=2
    n!/r!(n-r)! =15...
    I dont get it:scared:

    PLEASE HELP! my exam is next week and I still don't know how to solve these:eek:
     

    jay47

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      Just a general rule of thumb, when doing the math probability questions, try to think about them logically. Don't just jump in and try to plug in equations.

      For example, do what the guy above me did: she can choose 6 books at first, but because she already chose another, she could only choose 5 the second "event". It's really easy if you just think about it that way.
       

      eatkabab

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        the first one you got it correct. you've got the right equation, just takes a little working. you should be able to do it in your head in about a minute.

        average is 9, subtract from each, you get 6,3,0,9, square each and add, you get 126, divide by n-1. but I do see one problem, the answer should be 9,42 not 9,31.5. to get 31.5, you divide by n, or, 4. to get the standard variance, you divide by n-1, which would be 3. 126/3 is 42...



        as for the second problem, this is the difference between a permutation and a combination.

        for a permutation, the order you choose your items doesn't matter. this is the case with the question you're asking. it does not matter how you select the two books, you just select them. the eqtn for this is n!/(n-r). it would be 6!/4! which is 6X5, or, 30.

        a combination is a little more tricky and difficult to spot. this is when the order you choose your items in DOES matter. for example:

        you have 3 red balloons, 4 yellow, and 5 blue, how many ways can you choose 3 balloons?

        so the eqtn for a combination is what you have, n!/[(r!)(n-r)!]. that extra r! in the denominator makes all the difference. so the answer to this question would be 12!/[3!(9!)], or, 220... without that extra r!, you'd get 1320 which is the wrong answer.

        GOOOOOD LUCK!!!

        oh and if you can help me out, I'm looking for what the DAT is talking about when it asks for Descriptive and Experimental embryology. particularly implanting certain cells into a chicken or rat embryo and having the animal grow weird stuff... where do I learn this **** from?!

        by the way, my exam is on the 30th... :-(
         
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        Streetwolf

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          You actually don't need an equation for #1 to get it right.

          You are multiplying each number in the set by 3. Since the mean is a linear function, you multiply the original mean by 3 and you will get the new mean. And sure enough, 3x3 = 9.

          Since variance is a quadratic function, you just multiply the original variance by 3^2 = 9. And sure enough, 3.5x9 = 31.5.
           
          Sep 24, 2015
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            the first one you got it correct. you've got the right equation, just takes a little working. you should be able to do it in your head in about a minute.

            average is 9, subtract from each, you get 6,3,0,9, square each and add, you get 126, divide by n-1. but I do see one problem, the answer should be 9,42 not 9,31.5. to get 31.5, you divide by n, or, 4. to get the standard variance, you divide by n-1, which would be 3. 126/3 is 42...



            as for the second problem, this is the difference between a permutation and a combination.

            for a permutation, the order you choose your items doesn't matter. this is the case with the question you're asking. it does not matter how you select the two books, you just select them. the eqtn for this is n!/(n-r). it would be 6!/4! which is 6X5, or, 30.

            a combination is a little more tricky and difficult to spot. this is when the order you choose your items in DOES matter. for example:

            you have 3 red balloons, 4 yellow, and 5 blue, how many ways can you choose 3 balloons?

            so the eqtn for a combination is what you have, n!/[(r!)(n-r)!]. that extra r! in the denominator makes all the difference. so the answer to this question would be 12!/[3!(9!)], or, 220... without that extra r!, you'd get 1320 which is the wrong answer.

            GOOOOOD LUCK!!!

            oh and if you can help me out, I'm looking for what the DAT is talking about when it asks for Descriptive and Experimental embryology. particularly implanting certain cells into a chicken or rat embryo and having the animal grow weird stuff... where do I learn this **** from?!

            by the way, my exam is on the 30th... :-(

            you mixed up permutation and combination... in a combination order doesn't matter whereas in a permutation order does matter (but your formulas are correct)
             

            Halimaw

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            Dec 18, 2015
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              I know this is frustrating, because there shouldn't be inconsistencies in math, but there are two equations used for variance:
              S^2= sum (x-average)^2 / n-1 This is the first.
              S^2= sum (x-average)^2 / n This is the second.
              DAT tests use the second. The same goes for the standard deviation equation. Use "n" in the denominator, not "n-1"
              126/4=31.5
               
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