prompt: Yuan is 7 years older than his brother David and twice his age. How old is David?
messages.content: Yuan is 7 years older than his brother David and twice his age. How old is David?
messages.role: user
messages.content: Let x be David's age. Yuan is seven years older than David, 7 + x = Y years.
  Yuan is also twice David's age, 2 * x = Y years.
  Since these two equations equal the same thing, we can write: 7 + x = 2 * x.
  Subtracting x from both sides we get 7 = x, so David is 7 years old.
  #### 7
messages.role: assistant