A health department estimates that 12 vials of malaria serum will treat 150 people. At this rate, how many vials are required to treat 225 people?

This is a proportion problem. 12 vials needed for 150 is to x vials for 225 people.

12/150= x/225
Cross multiply
(150*x)= (12*225)
150x=2700
Divide both sides by 150
x= 18 vials

Check:
12/150=18/225
Divide
0.08=0.08 check

Hope this helps! :)