The relationship between 5 tablespoons of cocoa per 2 cups of milk is proportional(multiplicative). Let's represent the amount of tablespoons of cocoa we need for 1 cup of milk with x.
We can divide 5 by 2 in order to get x tablespoons for 1 cup of milk.
5 / 2 = 2.5
So, you need 2.5 tablespoons of cocoa for 1 cup of milk :)