The problem is mirrored: the dashed line shows where the mirror stands.
Mirroring flips both things - the order of the characters and every single digit.
So read from the mirror line towards the left and the problem is the right way round again.
Example
- 19
- 18
- 19
- 20
For parents
What does this exercise practice?
Mirror numbers train careful looking and the insight that a number does not lose its meaning just because it is turned around.
Common pitfalls
It works best if you read the whole problem first and only then calculate. Starting to calculate straight away often means mixing up the two numbers in subtraction.
How to practice at home
Write a problem on a piece of paper and hold it up to a real mirror - you can see right away what happens.