getneonow wrote:CLASS -> AVG AGE -> NO. OF STUDENTS
A -> 15-> 6
B -> 16 -> 12
Is the standard deviation of ages of students in class A greater than that of class B ?
1) The difference between ages of any two students in class A is always more than 1 yr.
2) No student in class B is more than 6 months older than any other student.
Even if you don't understand standard deviation, you should quickly be able to eliminate choices A, B and D, since clearly neither statement is sufficient alone (each statement only gives info on 1 of the 2 classes; we need info on both to solve).
So, let's chat about the question, then jump directly to combining the statements.
Standard deviation is the measure of how spread out the numbers in a set are from the mean: the greater the spread from the mean, the higher the standard deviation of the set.
To calculate the SD of a set (which you'll never need to do on the GMAT, so you definitely don't need to know the formula), you need to know two things about the set:
1) the number of terms; and
2) the exact spacing of all of the terms.
In this question we don't need to actually calculate the SD of either set - we merely need enough information to compare the two SDs.
On to the statements!
1) each term in the set is more than 1 unit apart from the number below it and the number above it.
2) the maximum range of class B is .5 units.
(1 unit = 1 year)
Based on (1), we know that class A contains 6 students and a range greater than 5; based on (2) we know that class B contains 12 students and a range less than .5. Accordingly, class B is packed much tighter around the mean and has a considerably lower SD than class A. Sufficient, choose (C).
