Try and adjust. Students guess an initial number, add and subtract through all the changes, and check to see if the end numbers match. If the numbers do not match, some students adjust the initial number up or down to compensate. Others try guessing initial numbers randomly. Undo net change.
Some students find the total in and total out and compute the net change.
By this method, students can manage problems with many changes. Undo from end to beginning. Four items were in the bag at the end. Two were added to get 4, so you must have had 2 before that. Whatever way they figure out the starting number, most children check their guesses by repeating the story, as shown in figure 6 .