conversations.from: human
conversations.value: With her savings, Sara bought two books: a book for 5.5£ and a book for 6.5£. She gives a 20£ bill to the seller. How much change does she get back?
  How to solve this?
conversations.from: assistant
conversations.value: To find out how much the saleswoman should return to her, we have to find out how much Sara spent in total. She spent 5.5£ + 6.5£ = 12£.
  So she gives 20£ and the cashier should keep 12£. The cashier therefore returns the difference between these two sums: 20£ – 12£ = 8£