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Mean mark math question

  1. Jun 2, 2005 #1
    i am having trouble with this question....

    On a test, the marks were normally distributed with a mean mark of 54 and a standard deviation of 9. If the number of students who received a mark less than or equal to k was twice the number of those who received a mark of 80 or better, then the value of k, correct to the nearest whole number, was......
  2. jcsd
  3. Jun 2, 2005 #2


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    What have you done? You can calculate the "standard" variable corresponding to x= 0: z= (80- 54)/9 and look up the probability in a table. (There's probably one in your textbook but http://people.hofstra.edu/faculty/Stefan_Waner/RealWorld/normaltable.html
    has one on the internet.) That's not actually the "number of students who received a mark of 80 or better"- it is actually "1- (number of student who recieved a mark of 80 or less/total number of students)". But subtracting it from 1 gives "number of students who received a mark of 80 or better/total number of students" and doubling that will then give "number of students who received a mark of k or less/total number of students" and then use that with the table to determine the corresponding "z" and finally solve for "x" (which, to the nearest integer, is your "k").
  4. Jun 2, 2005 #3
    i figured it out....thanks....
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