At the top there are five numbers, below them three cards. Each card is a rule, for example "greater than 400" or "digit sum 12".
Exactly one of the five numbers fits all three rules. It goes into the box on the right.
Take one card at a time and cross out the numbers that no longer fit. After three cards one is left - and every card is needed for that.
For parents
What does this exercise practice?
Test cards practice ruling things out. Instead of checking five numbers one by one, the child makes three passes and crosses out several numbers at a time. Along the way place value, digit sums and divisibility show up as properties of a number, not as sums to do.
Common pitfalls
Starting with the first number and checking all three rules takes fifteen tests in the worst case. The other way round - rule one on every number, then rule two on what is left - it is usually fewer than eight.
How to practice at home
One person thinks of a number up to 1000, the other may ask three yes-no questions - but only ones that work for any number ("are you even?", not "are you 372?").
Related: Number riddles, Even and Odd