Hi sunaimshadmani,
As a math subject, Standard Deviation comes from the broader category of statistics. It's essentially about how "spread out" a group of individual numbers is, relative to the average of that group. In real life, calculating the Standard Deviation of a group of numbers involves a big calculation (and you probably would NOT want to do that calculation by hand). Thankfully, the GMAT will NEVER ask you to actually calculate the Standard Deviation of a group of numbers. You're likely to be tested on the CONCEPT though.
In this DS question, we're told that there are two sets of numbers (with 5 numbers in each set). We're asked for the difference in the Standard Deviations of 2 sets of numbers.
Fact 1: Set A consists of consecutive multiples of 3.
The Set {3, 6, 9, 12, 15} would have the same SD as any other set of 5 consecutive multiples of 3 (for example {51, 54, 57, 60, 63}0, so we COULD figure out the SD of this set, but we don't know the SD of the Set B.
Fact 1 is INSUFFICIENT
Fact 2: Set B consists of consecutive multiples of 5.
This is the same situation as in Fact 1 above (we would just be using consecutive multiples of 5), but we don't know the SD of Set A.
Fact 2 is INSUFFICIENT
Combined, we know we have...
Set A = 5 consecutive multiples of 3
Set B = 5 consecutive multiples of 5
We COULD calculate the difference in the two SDs!
Combined, SUFFICIENT.
Final Answer:
GMAT assassins aren't born, they're made,
Rich
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