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From 1 and question S: (30+40+50+x+25)/5 = 40
=> x = 55

=> numbers are: 25,30,40,50,55

From 2: (30+40+50+p+45)/5 = 40

=> numbers are: 10,25,30,40,50

Together, this seems to imply T has the higher standard deviation. Either way, we dont need to know exactly, just the fact that we have all the numbers to do the calc is enough

C
From 1)
Set S - 25, 30, 40, 50, 55 (calculate the last one from average)
We know the SD of Set S but donâ€™t have any idea about the SD of T

From 2)
Set T - 30, 35, 40, 45, 50.

Together
30, 40 and 50 are common members. Other elements of these sets will decide which set has greater SD.
new elements of Set S are further away from the mean (40).
SD of Set S > SD of Set T

get the mean
get the difference of the mean and each member of the set
square the differences
average the squared differences
take the square root of that average

i think GMAT rarely asks you to compute for SD. just tests the concept _________________

For all SD questions you need to know terms in an individual series in order to be 100% sure.
Any information on mean median mode or range is not sufficient.

From st1 and given average we can determine ther terms in series one
From st2 and given average we can determie the terms in series 2

Individually 1 and 2 are not suff so eliminate A B and D