statistics
posted by Barbara on .
Assume that the mean hourly cost to operate a commercial airplane follows the normal distribution with a mean of $2,225 per hour and a standard deviation of $220.
What is the operating cost for the lowest 5 percent of the airplanes?

According to the standard bell curve and the empirical rule, the lowest 5% would be 2 standard deviations below the mean making the operating cost of the lowest 5% of airplanes equal to $1785

Z = (scoremean)/SD
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the Z score related to that proportion.
For the lowest 5%, you need a Z score of 1.645.
Insert the values in the above equation to solve for the score. 
.59

Assume that the hourly cost to operate a commercial airplane follows the normal distribution with a mean of $5,018 per hour and a standard deviation of $493.
What is the operating cost for the lowest 1% of the airplanes? (Round z value to 2 decimal places and round final answer to nearest whole dollar.)
Operating cost