Let's assume the number of $20 bills is x.
Since the number of $10 bills is twice the number of $20 bills, then the number of $10 bills is 2x.
The value of the $20 bills is $20 * x = $20x
The value of the $10 bills is $10 * 2x = $20x
The total value of the bills is $20x + $20x = $40x
Since the total amount in the fund is $120, then $40x = $120
Dividing both sides by $40 gives x = 3
Therefore, there are 3 $20 bills in the fund.
#### 3
The answer is: 3
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
If the total amount in Karen's class fund is $120, consisting of only $10 and $20 bills, and the number of $10 bills is twice the number of $20 bills, how many $20 bills are there in the fund?

### Answer:
