A batch cupcake recipe calls for 60 teaspoons of baking powder, but all you have is baking soda. The baker can substitute two parts cream of tartar and one part baking soda for the baking powder. How many teaspoons of cream of tartar are needed?

2/3 of the teaspoons are cream of tartar, since for every 2 teaspoons of cream of tartar you also have 1 teaspoon of baking soda. 2/3 of 60 is 40.

(2/3)(60) = 40 teaspoons

As a ratio equation:

2/3 = x/60