Write down all the numbers in the range and cross out what does not fit for each machine.
Important: a 'not true' helps too! "The number is even - not true" means all even numbers are out.
In the end exactly one number is left. Every machine is needed for that.
Example
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
"Not true" 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?