What is happening
You arrive at a library trolley that has to go back to the shelf full.
The trolley cannot go back half empty. There are 3 books on it, and the shelf slot it belongs to takes 9.
Counting on to nineRunning
Books on the trolley
12books
Slot takes
9books
Books on the trolley is over by 3 books
Sequence steps. Books added at 9 books. Books on the trolley 12 books, Slot takes 9 books. Currently books on the trolley is over by 3 books.
What has to balance