You walk into a fast food restaurant which is so empty that there is a member of staff waiting to take your order. You know what you want but you don’t know how much it will cost. As you give your order your eyes take in the prices of the items you want: one burger £1.49, another burger £1.69, one portion of fries £0.89, one cold drink £0.79, one hot drink £0.59. You want to work out roughly how much it will be so you can decide whether to count up your change or get out a note.