Write down all the numbers in the range and cross out what does not fit for each machine.
Important: a cross helps too! "The number is even - cross" means all even numbers are out.
In the end exactly one number is left. Every machine is needed for that.
Example
- 1
- 8
- 15
For parents
What does this exercise practice?
Test machines show that a denied statement tells you just as much as a confirmed one - the heart of all detective logic.
Common pitfalls
The cross is often overlooked or read as "don't know". It is a clear statement though: this property does not apply.
How to practice at home
One person thinks of a number up to 20, the other asks yes-no questions. How few questions are enough?