Greetings!
You can create an equation to solve for x:
[tex]9/10=x/400[/tex]
Cross Multiply.
[tex]9*400=10*x[/tex]
[tex]3600=10x[/tex]
Divide both sides by 10.
[tex](3600)/10=(10x)/10[/tex]
The Answer Is:
[tex]360=x[/tex]
Out the 400 smokers, approximately 360 of them started smoking before age 21.
Hope this helps.
-Benjamin
