Question

Suppose X is a random variable with mean μ and standard deviation σ. If a large number of trials is observed, at least what percentage of these values is expected to lie between μ − 3σ and μ + 3σ? (Round your answer to the nearest whole number.)

Answer 1

Look in the back of your statistics textbook for a table called something like “area under normal distribution” to find the proportion/probability of Z = ±3.

Or you can use this:

http://davidmlane.com/hyperstat/z_table.html