conversations.from: human
conversations.value: Karen's class fund contains only $10 and $20 bills,  which amount to $120. The number of $10 bills is twice as many $20 bills. How many $20 bills do they have in their fund?
  Give me a solution to this problem
conversations.from: assistant
conversations.value: If n is the number of $20 bills then 2n is the number of $10 bills.
  So the equation will be $10(2n) + $20n = $120.
  By combining like terms, the equation becomes 40n = 120.
  So, the value of n which represents the number of $20 bills is 120/40 = 3.