conversations.from: human
conversations.value: Gerald had 20 toy cars. He donated 1/4 of his toy cars to an orphanage. How many toy cars does Gerald have left?
  Solve it.
conversations.from: assistant
conversations.value: Gerald gave away 20 x 1/4 = 5 of his toy cars.
  Therefore, he is left with 20 - 5 = 15 toy cars.