:9a. Solution.  Consider the deviations from the average.  40%
of them are equal to 20-15 = 5, 30% to 15-15 = 0, 20% to 10-15 =
-5, and 10% to 5-15 = -10.  As for the square deviations, 40% +
20% = 60% of them are equal to 25, 30% to 0, and 10% to 100.  The
average of the square deviations is equal to <p>

(60x25 + 30x0 + 10x100)/100 <p>

= (1200 + 1000)/100 <p>

= 2200/100 <p>

= 22
 
The SD is equal to the square root of 22, or 4.69.  The closest
of the answers is 4.5 <p>