A scientist recorded the number of eggs in each of 10 birds'

Hi jjjinapinch,

To start, Standard Deviation is a relatively rare category on the GMAT (you'll likely see just one question that asks you about it) - and you'll never be asked to calculate S.D. using the S.D. Formula. As such, you'll really only be tested on the 'concept' of S.D. (and S.D. is basically a measure of how 'spread out' a group of numbers is).

We're told that a scientist recorded the number of eggs in each of 10 birds' nests. We're asked for the standard deviation of the numbers of eggs in the 10 nests.

1) The average (arithmetic mean) number of eggs for the 10 nests was 4.

With the average number of eggs, we know that there are a total of (10)(4) = 40 eggs, but we don't know the individual number in each nest (so we don't know how 'spread out' the numbers are).
Fact 1 is INSUFFICIENT

2) Each of the 10 nests contained the same number of eggs.

When all of the numbers in a sample are the SAME, that means that there is NO 'spread' and the S.D. is 0. With the information in Fact 2, we can answer the question.
Fact 2 is SUFFICIENT

Final Answer: B

GMAT assassins aren't born, they're made,
Rich