Question

a cookie recipe state for every 3 cup of flour 1 1/2 teaspoons of vanilla are needed how many teaspoon are neededfor 5 cup of flour

Answer 1

First you divide 5/3 to determine how many total 1.5 teaspoons of vanilla are needed.  So then you will do (5/3)(3/2) which gets you 5/2 teaspoons of vanilla or 2.5 teaspoons.