This shows a figure containing a doing and undoing diagram. The doing diagram has a rectangular box in the middle containing ‘subtract 3’. To the left of this is the word number, linked to the rectangular box by an arrow pointing right. On the right of the rectangular box is an arrow pointing right to the number 2.

The undoing diagram has a rectangular box in the middle containing ‘add 3’. To the left of this is the number 2, linked to the rectangular box by an arrow pointing right. On the right of the rectangular box is an arrow pointing right to the number 5.

3.1 Doing some doing and undoing