For both Montessori and non-Montessori classrooms to teach the concept of a Fact Family with addition and subtraction as inverse operations. I present initially along with 3 bead bars (can be any objects) for students to see the concrete representation of the concept.
Teacher can choose numbers to start (in the 3 circles) and then child can learn to do it independently. If the child struggles with choosing three fact family numbers, have him/her fill in the first addition problem and then use those three numbers.
Please see my Simple Fact Family Activity - Multiplication/Division.