1/3 cups is for 6 servings how much do I need for 20 servings

Set up a proportion:

(1/3) cups over 6 servings = x cups over 20 servings. $${\frac{\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right)}{{\mathtt{6}}}} = {\frac{{\mathtt{x}}}{{\mathtt{20}}}}$$

Then, cross multiply:

$$\left({\frac{{\mathtt{1}}}{{\mathtt{3}}}}\right){\mathtt{\,\times\,}}{\mathtt{20}} = {\mathtt{6}}{\mathtt{\,\times\,}}{\mathtt{x}}$$

Multiply it out:

(20/3) = 6x

Divide both sides by 6. Dividing by 6 is the same as multiplying by (1/6)

 x = (20/3) * (1/6)

x = 20/18

Simplify. 

x = 10/9 or 1.11111111...cups.