Question

It is estimated that amounts of money spent on gasoline by customers at their gas stations follows a normal distribution witha standard deviation of $3. It was also found that 10% of all customers spent more than $25. What percentage of customers spent less than $20.

Answer 1

Z = (score-mean)/SD = (25-mean)/3

Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion (.1) that corresponds to a Z score.

Use that Z score in the above equation to find the mean. Then use that mean and Z score to find the new percentage.