Hi sharma215,

For future reference, you should list just one question per post. This will keep the discussion focused on one subject and reduce any confusion about criss-crossing posts on different prompts.

Standard Deviation is essentially about how "spread out" a group of numbers actually is. When SD is small, the group of numbers is relatively "close" together. When SD is big, the group of numbers is more spread out. It's not a big subject on the GMAT (you'll likely see it just once), but it's based on some rules that you can learn.

For Q1, we're given the mean/Average of a group (75) and that group's SD (10). The question asks what values fall within 1 SD of the mean. Standard Deviations go "up" and "down", so the range of values that fall within 1 SD of this mean is...

75 - 10 = 65
75 + 10 = 85

Range = 65 to 85

For Q2, I think there's something missing. Did you transcribe this question correctly?

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

A little extra background on standard deviations above and below the mean

If, for example, a set has a standard deviation of 4, then:
1 standard deviation = 4
2 standard deviations = 8
3 standard deviations = 12
1.5 standard deviations = 6
0.25 standard deviations = 1
etc


So, if the mean of a set is 9, and the standard deviation is 4, then:
2 standard deviations ABOVE the mean = 17 [since 9 + 2(4) = 17]
1.5 standard deviations BELOW the mean = 3 [since 9 - 1.5(4) = 3]
3 standard deviations ABOVE the mean = 21 [since 9 + 3(4) = 21]
etc.

Cheers,
Brent