In a certain population of clover, the number of flowers on each plant is approximately normally distributed with a mean of 10.8 flowers/plant and a standard deviation of 2.1 flowers/plant. What percentage of the plants fall between 8 and 11 flowers?

Answer 1

Use David's excellent "normal distribution" calculator

http://davidmlane.com/hyperstat/z_table.html

click on "area from a value"
enter the data, click on "between" and enter 8 and 11 to get .4467

You can enter the data directly, no need to find z-scores first.
If you have converted to z-scores, enter 0 for mean and 1 for sd, your input for 8 would be -1.3333
and the input for 11 would be .095238
to get the same result of .4467