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help me with this standard deviation question

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[QUOTE=sharma1617;808316]The fourth grade at school X is made up of 300 students who have a total weight of 21,600 pounds.
If the weight of these four graders has a normal distribution and standard deviation equals 12 pounds, approximately what percentage of the fourth graders weighs more than 84 pounds?

a 12%
b 16%
c 36%
d 48%
e 60%

need detailed explanation,,and also want to know what are bell curve questions and how it will relate with standard deviaion. thanks[/QUOTE]
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  1. shabbirahsan's Avatar
    The mean is 21600/300 = 72 pounds. SD is 12 pounds. According to rule, if it is normally distributed +- 1 SD covers 68% of the data meaning 72+_12 = a range of 60 to 84 pounds children make up 68 % of the school. So rest 32% are more than 84 pounds. and less than 60 pounds. since, it is normally distributed 32/2 = 16% children are above 84 pounds.