Normal Distribution in I am so confused with this problem:
The middle 95% of adults have an IQ between 60 and 140. Assume that IQ for adults is normally distributed. 
a. What is the average IQ for adults? The standard deviation?
I got the average by subtracting the values given and then multiply it with 95%. But I dont know how to get the standard deviation because a certain number of population isn't given. Any ideas anyone?

Actually the average IQ is 100 and its standard deviation is 15.
Intelligence tests are scored in such a way the resulting IQ distribution conform to these properties.
http://en.wikipedia.org/wiki/Intelligence_quotient 
