# Math Help - Normal Distribution HELP!

1. ## Normal Distribution HELP!

Hello, I don't understand how the NORMAL DISTRIBUTION column of the attached chart is calculated, can someone please explain?

The mean is 40 and the Standard Deviation is 10

3. ## Re: Normal Distribution HELP!

For the row 20<30 they find what percent of the population lies between 20 and 30 in a normal distribution. If you look at a normal distribution table like this one Standard Normal Distribution Table
you can see what percent is between a given number of standard deviations from the mean.
20 is 2 standard deviations below the mean $\frac{20-40}{10}=-2$ and 30 is 1 standard deviations below the mean $\frac{30-40}{10}=-1$
From the table you can see that 34.1% lies between the mean and 1 standard deviation below the mean. Also 47.7% lies between the mean and 2 standard deviations below the mean. Therefore 47.7-34.1= 13.6% lies between 1 and 2 standard deviations below the mean. 13.6% is the 0.1359 figure in the table.