Posted by **Barbara** on Sunday, November 21, 2010 at 3:44pm.

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?

- statistics -
**Joe**, Sunday, November 21, 2010 at 6:45pm
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

- statistics correction -
**PsyDAG**, Monday, November 22, 2010 at 8:32am
Z = (score-mean)/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.

- statistics -
**m**, Thursday, April 18, 2013 at 9:30pm
.59

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