suppose a normal distribution has a true population mean of 18 and a true standard deviation of 2. what % of observations will be 18 or less, 18 or greater and in the range from 16 to 20?
Z = (score-mean)/SD
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability for the Z scores. Multiply by 100 for percentages.