A few months ago Mr and Mrs Jezza Bear went out for the day to the city. Whilst doing the shopping they
decided to stop at a pleasant cafe in the heart of the old town. They decided to have a nice cool drink each
and some tasty pastry snacks. The waiter tells them that the pastry snacks are only served in portions of
6, 9 or 20.
The question: What is the biggest number of pastry snacks that cannot be ordered in these portions?