Turn 2 1/2 into an improper fraction by multiplying 2 by 2, then adding 1 to that number and using 2 as the new denominator. You get 5/2. Then make it the same denominator as the serving size by multiplying it by 2, or 2/1. You get 10/4. After that, see how many times 3 goes into 10. It’s 3, with 1 remaining. Therefore, he can make 3 1/4 servings, or 3, if you need an exact answer.
