Your father is planning to cook for 25 people using a recipe that specifies two-thirds cups of sugar for every six people.

For the larger group,

How many cups of sugar will your father require

Let's say each 2/3 cups is X

and X=6

4x=24

Now we need 1/6 of 2/3 which is 2/18

So you need 4 and 1/9

Yay!