?
I did not understand.
Any one can be more didatic?

MTeles wrote:?
I did not understand.
Any one can be more didatic?

MTeles, though I agree with the pro's on here that this is a bad question and lacks a few points, here is how to answer this one.

So I figured this one out... it's definitely a tough one and I wouldn't recommend trying to spend time to solve it out on the actual exam. However, knowing that these pieces of information will find you the answer could help on the exam.

Anyhow, here's how to answer this one...

I'm going to skip past A) and B), because it's clear that they don't provide enough info. So by combining them, we get the following:

10 of the 500 recruits scored 82 or higher. 10/500= 2%

Now, lets take a look at a Normal Distribution chart:



This graph gives you a good idea of what % is covered by each "block" of standard deviation (1 up and 1 down). The general rule on this for the GMAT is that 1 standard deviation covers 68%, 2 cover 95%, 3 cover 99%. As you can see in the chart above, standard deviations cover the variance above and below. The chart above has slightly different percentages, but helps for you to understand the logic I'm about to make.

Point A) So, 1 standard deviation above equals ~34%, 2 above equals ~13.5%.. (95%-68%)/2, and 3 above covers ~2%... (99%-95%)/2. The rest, is covered by .5%

Point B) Since the mean is 72 and 3 standard deviations from the mean begins at 82, there are two standard deviations between 72 and 82. The difference here is 10, so each standard deviation represents a range of 5.

Point C) That means that 1 standard deviation covers 72-77, 2 standard deviations covers 77-82... and the opposite logic can be applied to the lower end of the data (-1 sd = 67-72, -2 sd = 62-67, and -3 = 57-62).

Point D) Now lets take a look at how the bottom 16% correlates to standard deviation. Per "point A", the bottom ranges cover ~13.5%, ~2%, and ~.5%. Add these three together and you get the bottom 16%. So, lets go back to "point c" and we get that this 16% falls into the score range of anything below 67 (anything below the 67-72 range of -1 sd). There's your answer!

I agree with others that this is a poorly stated question and makes you assume a few points (suchs as the approximate percentages that I gave for range of standard deviation), but this general logic will help if a similar question comes up on your exam. [/img]