The term "grading on a curve" is often heard. When teachers assign grades based on the notion that grades should be normally distributed, grades are assigned as follows:
Grades with 1 SD of the mean earn a C
Grades between 1 SD above the mean and 2 SD above the mean earn a B.
Grades greater than 2 SD above the mean earn an A
Grades between 1 SD below the mean and 2 SD below the mean earn a D
Grades greater than 2 SD below the mean earn an F
If grading is done in this manner for a very large lecture class containing 400 students, how many of them would earn a C? How many would earn an A?

Question

The term "grading on a curve" is often heard. When teachers assign grades based on the notion that grades should be normally distributed, grades are assigned as follows:
Grades with 1 SD of the mean earn a C
Grades between 1 SD above the mean and 2 SD above the mean earn a B.
Grades greater than 2 SD above the mean earn an A
Grades between 1 SD below the mean and 2 SD below the mean earn a D
Grades greater than 2 SD below the mean earn an F
If grading is done in this manner for a very large lecture class containing 400 students, how many of them would earn a C? How many would earn an A?

pphalke · Answer

please help @Photon336

zarkon · Answer

If $Z$ is a standard normal random variable, what is 

$$P(-1\le Z\le 1)$$

pphalke · Answer

no idea

zarkon · Answer

Are you using a normal table in class or a calculator/computer to calculate probabilities?

pphalke · Answer

calculator but it still difficult to me

zarkon · Answer

are you using a TI 83/84 calculator?

zarkon · Answer

attachment