a recipe of 20 bread rolls requires 5 tablespoons of butter. How many tablespoons of butter are need

a recipe of 20 bread rolls requires 5 tablespoons of butter. How many tablespoons of butter are needed for 30 bread rolls?

Set up a proportion of bread rolls per tablespoons of butter where t is the amount of tablespoons of butter needed for 30 bread rolls:
20/5 = 30/t

Cross multiply our proportion:
Numerator 1 * Denominator 2 = Denominator 1 * Numerator 2
20t = 30 * 5
20t = 150

Divide each side of the equation by 20:
20t/20 = 150/20

Cancel the 20's on the left side and we get:
t = 7.5