The vending machine gives customers the choice of paying either 40 or 50p. However, the amount they pay will not change the amount of sweets they get, instead it will change the amount of sweets the next customer gets. the machine shows you how many sweets you get.

Every time a customer pays 50p, the quantity of sweets served will increase for the next customer and so on. The opposite will happen when a customer pays 40p.

The result being that if people work together, they will always get more smarties than they paid for, if not they will only get what they pay for.

if a lot of people have been
paying 40p, there won't be
any bonus sweets available
for you. the amount will seem
just reasonable for a vending
machine.

if a lot of people have been
paying 50p, you will get the
maximum amount of sweets,
which is a bargain, even for
50p.