A compact disc is designed to last an average of 4 years with a standard deviation of 0.9 years. What is the probability that a CD will last less than 3 years?
3 is 1.0 below the mean. That is 1.0 / 0.9, or 1.1111... standard deviations below the mean. So look up z < -1.11 on your graphing calculator. It should be around 0.1335 or 13.35%
