for a normal distribution problem:

mean = 100
standard deviation = 10

80% of the values are between what 2 x values symmetrically distributed around the mean.
How can you find the answer?

80% of the data is between what two x-values? Well, that means 40% is to the left of the

mean and 40% is to the right of the mean.

Draw the bell-curve. That will help. If 80% is in between, then 10% lies to the left and 10%

to the right of the said area.

Look up 0.10 in the body of the table and see what z-score corresponds. Look up 0.90 in

the body of the table and see what z-score corresponds.

Then use the formula:

z=\frac{x-{\mu}}{\sigma} and solve for x.

Of course, {\sigma}=10 and {\mu}=100