Well you can't actually find a number, but you can make an equation.
lets call the bag of candy x, and the number of students in the teachers class S.
she gives every student in her class 4 pieces, so to find that amount, we have to multiply the number of students in her class by 4, the pieces of candy. 4S
afterwards she has 48 pieces, which is the endpoint, x is the starting point because that's the amount of candy the teacher started with.
so your equation is x-4S=48, but the thing is, the question is looking for the amount of students she gave candy to, so we have to isolate S, or in easier words; put S on one side, and the numbers on the other.
so the equation is S=1/4x-12
