An exercise to practice writing requirements (from Sommerville's text: Ex. 4.5 p. 116 of 9th ed. or Ex. 4.5 p. 122 of 10th ed. or Ex. 6.6 p. 141 of 8th ed.):

As suggested in this chapter (ch. 4 of 9th or 10th ed. or ch. 6 of earlier editions), where natural language is presented in a standard way, write plausible user requirements for the following function:

An unattended petrol (gas) pump system that includes a credit card reader. The customer swipes the card through the reader and then specifies the amount of fuel required. The fuel is delivered and the customer's account debited.