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!