Several roads lead from the castle to the tower. Each one stops at a resting place.
At every sign you pay a toll. On one road you pay both of its signs together.
On the road with the smallest sign you do not pay the least - only adding up reveals it.
Example
For parents
What does this exercise practice?
The road map practices forming several sums and comparing them, instead of going by the smallest single number.
Common pitfalls
Children like to pick the road with the smallest single number. That shortcut is deliberately misleading here - only both signs together decide.
How to practice at home
Draw two places on a sheet of paper, sketch several roads between them, write a toll on each leg, and then hunt together for the road where you pay the least.