The proportion would be -
12 / 150 = x / 225.
That is, 12 per 150, x amount per 225
To solve this proportion, you can cross multiply (multiply the values diagonally) -
12 / 150 = x /225
150x = 12(225)
150x = 2700
x = 18
18 vials are required to treat 225 people.
Hope this helps! (: