gmatmachoman wrote:
This one really test the fundae of SD
Rule :When the relative change in entities in a list is SAME , then SD remain SAME
St1 :For each tank, 30% of water at the beginning was removed
As per the rule , SD is same as 10 units
Sufficient
No, this is not the case. If you reduce all of the quantities by 30%, *all* of the distances between quantities go down, by exactly 30%. So the standard deviation will *not* stay the same; it will drop by 30%. The answer is still A, because if we know the initial standard deviation is 10, we know the new standard deviation will be 7.
You might be thinking of a situation where we simply *add* or *subtract* a number from everything in a set. Here, if we simply removed 1 gallon of water from each tank, then all of the distances in our set would remain unchanged. Then the standard deviation would not change at all; it would still be 10. But if we *multiply* each element in our set by a constant, as in this question (we are multiplying by 0.7), we are changing all of the distances in the set, and this will certainly change the standard deviation.
