Step by step
The question
Question: the numbers are normal, with mean 50 and SD 10: what percent is below 20?
Normal means the numbers make a bell shape, the same on both sides of the mean.
1 Draw the bell
The mean, 50, is in the middle.
Each mark is one SD, 10, further: 50 plus 10 is 60, then 70, then 80.
And going down: 40, 30, 20.
2 The 68-95-99.7 rule
The rule: 68 percent are within 1 SD of the mean.
95 percent within 2 SDs.
99.7 percent within 3 SDs.
Split into pieces, half on each side.
68 divided by 2 is 34.
95 minus 68 is 27, half is 13.5.
99.7 minus 95 is 4.7, half is 2.35.
And 0.15 at each end.
We write each piece on the curve.
3 Shade the part
20 is 3 SD below the mean.
Below it: everything to the left.
Careful!
Outside 3 SD is 0.3 percent, but that is both ends together.
One end is half of it.
4 Add the pieces
The shaded piece is 0.15%.
5 Check
Let's check: the pieces not shaded add up to 99.85 percent.
0.15 plus 99.85 is 100 percent: the whole bell.
So it is right!
Let's look back.
First: draw the bell.
Then: the 68-95-99.7 rule.
Then: shade the part.
Then: add the pieces.
Last: check.