Twenty-one students at a school have an allergy to peanuts, shellfish, or both. Fourteen students at the school are allergic to peanuts. Twelve students at the school are allergic to shellfish. How many of the students are allergic to both peanuts and shellfish? A 12 B 7 C 5 D 2

Add the 14 and the 12. You get 26, then subtract it by the total number of students, 21, then you get 5.