###Human: 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?, ### Assistant: 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. 
 So the answer is :  3