Question 1033517
<pre>
In a normally distributed population, by the 
"empirical rule",

1.  68% of the distribution lies within one standard
    deviation of the mean. 

2.  95% of the distribution lies within two standard 
    deviations of the mean. 

3.  99.7% of the distribution lies within three standard 
    deviations of the mean.

Therefore, in that library, we can conclude that: 

1.  68% of the books have between 180-30 or 150 and 
    180+30 or 210 pages.

2.  95% of the books have between 180-2*30 or 180-60 
    or 120 and 180+2*30 180+60 or 240 pages.

3.  99.7% of the books have between 180-3*30 or 180-90 
    or 90 and 180+90 or 270 pages.

Edwin</pre>