conversations.from: human
conversations.value: Mike wants to buy a new phone. The cost of the phone is $1300. How much more money does Mike need if he already has 40% of the amount he needs?
conversations.from: assistant
conversations.value: Mike already has 40/100 * 1300 = $520.
  This means he needs 1300 - 520 = $780 more, to buy the phone.