Challenge Level

Mr Gilderdale was thinking of a number rule. He asked the class to choose numbers to test so they could try to work out what the rule was.

If the number they chose fitted his rule, he wrote it on the board under 'I like these numbers'. If the number they chose didn't fit his rule, he wrote it under 'I don't like these numbers'.

After the class had chosen four numbers, this is what was on the board:

I like these numbers | I don't like these numbers |

15 | 18 |

5 | 22 |

What could Mr Gilderdale's rule be?

If you were in Mr Gilderdale's class, which number would you choose next to test your idea?

How could you find out Mr Gilderdale's rule in the smallest number of guesses?