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.