OK,
So let's just start with one liter. We know that 1/5 of a liter can fill 1 cup, making it so that 1 liter fill up 5 cups. We can set up the ratio 1:5, 1 being liters, and 5 being the amount of cups that can be filled from the amount of liters.
Now, because we have 8 liters, multiply 1 by 8, giving us 8 liters. What you do to one side of the ratio, you must do to the other to make the amount of liters to amount of cups that can be filled proportional. 5 times 8 would be 40, making the ratio 8:40, therefore giving you 40 cups that can be filled from 8 liters.
I hope you followed me...