Question

With a mean of 100 and a standard deviation of 15 and if one individual is selected, what is the probablity of that individual having an IQ between 85 and 115?

Answer 1

http://en.wikipedia.org/wiki/68-95-99.7_rule
As can be seen at the above link the empirical rule gives the percentage of values that lie between plus and minus one standard deviation from the mean. This percentage can be converted to a probability by dividing by 100 to give the answer.