Step by step
The question
Question: X is normal, with mean 65 and SD 4: what is the probability that X is more than 71.2?
μ is the mean.
σ is the standard deviation, the SD.
1 Find z
z counts how many SDs x is from the mean: x minus the mean, over the SD.
71.2 minus 65 is 6.2.
Divided by 4, that is 1.55.
2 Draw and shade
The bell: the mean, 65, in the middle.
71.2 is right of it.
We want X more than 71.2.
We shade that part.
3 Read the table
The z-table gives the area to the left of z.
Row 1.5, column .05.
For z = 1.55: the row 1.5 and the column .05 meet at 0.9394.
4 The area we want
Careful!
0.9394 is the area to the left.
We want the right side.
The whole bell is 1.
The right side is 1 minus the left: 1 minus 0.9394 is 0.0606.
5 Check
Let's check another way.
The bell is the same on both sides.
So the area right of 1.55 is the area left of −1.55.
The table at −1.55 gives 0.0606.
The same!
So it is right!
Let's look back.
First: find z.
Then: draw and shade.
Then: read the table.
Then: the area we want.
Last: check.