PART A
If 80% of smokers started before the age of 18, we would expect 80% of 400 to have started smoking before the age of 18. Keep in mind that percent means out of 100, so 80% is [tex] \frac{80}{100} [/tex]
We find 80% of 400 as follows: [tex](\frac{80}{100})(400)=(80)(4)=320[/tex]
PART B
Here we find that in a group of 400 there are 360 that started smoking before the age of 18. Let us find what percent that represents. Since percent means out of 100, we want to write 360/400 as a fraction with 100 in the denominator. That is, [tex] \frac{360}{400} = \frac{90}{100} [/tex]. That is 90% of the people in the sample started smoking before age 18. This is 10 percentage points higher than what we expect (80%) and so is unusual.
