conversations.from: human
conversations.value: Farmer Brown has 20 animals on his farm, all either chickens or cows. They have a total of 70 legs, all together. How many of the animals are chickens?
conversations.from: assistant
conversations.value: Let C be the number of chickens.
  There are 20-C cows.
  The cows have 4*(20-C) legs.
  The cows have 2C legs.
  The total number of legs is 2C+4(20-C)=70.
  2C+80-4C=70
  2C=10
  C=5