Question

Approximately 9 out of 10 smokers began smoking before the age of 21. In a group of 400 smokers, about how many of them started before they reached their 21st birthday?

Answer 1

400 / 10 = 40. 

     40
   X  9
= 360

In a group of 400 smokers, about 360 of them started before they reached their 21st birthday.