The way I understand this, is that we will have 3 (liters of water) +0.5 (half a liter of lemonade) =3.5 liters of liquid
so the question would be: how many times does [tex] \frac{1}{3} [/tex] fit into 3.5?
I count this like this:
[tex] \frac{3.5}{ \frac{1}{3} } = \frac{3.5*3}{1} =9+1.5=10.5[/tex]
if you want to divide by a fraction, you need to multiply by the reverse of it.
so the answer is: 10 servings of lemonade ( and a little bit will be left over for half a serving).