Display a picture or a slide of a whole pizza before it has been sliced. Ask the class how many pieces you need to cut to give everyone in the class a piece of pizza. Write this number on the board beside the pizza. For example, if you have 20 students in the class, write 20 on the board. This means you must cut the pizza in 20 equal pieces to give one piece to each student. Divide the pizza into 20 equal parts and write "20/20 = 1." Use a square or rectangular pizza to make your job easier.
Separate each of the pizza pieces and write "1/20" next to several of the pieces so students understand that one is how many parts you have and you divided the original pizza into 20 equal parts. Use additional pizza visuals to demonstrate that you have two pieces equal to 1/2 each and that 2/2 = 1 and so forth. In each case, the bottom number, or denominator, equals the number of pieces that make up the whole and the top number, or numerator, equals the number of pieces you currently have out of the whole.
Have one row of students each take a single piece of pizza and stand in a group. Count the number of students with pizza and write that number as the numerator and the number of pizza parts in a whole as the denominator. Give each student in another small group one piece of pizza each and write that number as a fraction. Show that because the denominator is the same, you can add the total number of students together to determine how many pieces you have removed from the pizza. You add only the numerators, not the denominators, because the number of pieces in the whole pizza didn’t change. Demonstrate this concept with pizzas cut into a small number of equal parts.
Explain that you subtract the total number of students who now have a piece of the pizza from the total number of pizza parts to determine how many pieces of pizza you have left. Tell the students that like adding fractions, you only subtract the numerators and not the denominators, because the total number of pieces making the whole pizza didn't change. Demonstrate subtracting like fractions using other pizzas cut in fewer equal parts.