The guard walks the castle wall from left to right, carrying torches.
On the battlement above he is given more, in the gap below he hands some over.
He must never run out on the way. The question is how few he needs at the start.
On the wall band you can note how many he still has after each step.
Example
For parents
What does this exercise practice?
The watch wall practices thinking ahead instead of just calculating. Adding up all the minus stones gives too much, the biggest minus stone alone usually too little. What counts is the tightest spot along the way, and the child only finds it by keeping the whole wall in view.
Common pitfalls
The most common mistake is adding up all the minus stones and ignoring the battlements. What the guard picks up on the way helps him, after all. Conversely, the biggest minus stone alone is not enough if torches were already used up before it.
How to practice at home
Build a wall from toy blocks and play it through with coins. First set off with too few coins and see where it gets tight, then take one more. That makes the tightest spot visible instead of calculated.
Related: Caterpillar, Maze