The guard walks along the castle wall, carrying torches.
Every battlement says how many he is given there (+) or has to hand over (−).
He must never run out on the way. The question is how few he needs at the start.
The guard on the left walks right, the guard on the right walks back - that makes two answers.
In the wall band you can note how many he still has after each step: the way there on top, the way back below.
Example
For parents
What does this exercise practice?
The watch wall practices thinking ahead instead of just calculating. Adding up all the minus battlements gives too much, the biggest minus battlement 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. The way back then shows that the tightest spot depends on the order - same wall, other direction, other answer.
Common pitfalls
The most common mistake is adding up all the minus battlements and ignoring the plus ones. What the guard picks up on the way helps him, after all. Conversely, the biggest minus battlement alone is not enough if torches were already used up before it. And the way back cannot be copied from the first answer - it has to be walked again.
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. Then walk the same wall from the other end. That makes the tightest spot visible instead of calculated.
Related: Math Caterpillar, Math Maze