This problem is an amusing and thought-provoking way of practising addition, subtraction, multiplication and division. The results are diverse enough to keep learners working for a considerable time and to encourage methodical working. This sheet might be helpful.

What operation do you need to do next?

Would it be a good idea to jot down your calculations as you go?

What numbers have you put in so far?

Have you made a list of the numbers you have already done?

How many numbers have you found that come out unchanged?

Learners could make their own number-crunching machine and try it out on their friends.