To solve this problem, we need to determine the value of x, which represents the number of $x bills in the class fund.
We know that there are 3 $20 bills in the fund. Therefore, the number of $x bills is twice as many as the $20 bills, which is 2 * 3 = 6.
Let's set up the equation based on the given information:
Number of $10 bills * Value of each $10 bill + Number of $20 bills * Value of each $20 bill = Total amount in the class fund
0 * $10 + 3 * $20 + 6 * $x = $120
Let's simplify and solve for x:
60 + 6x = 120
To isolate x, we subtract 60 from both sides of the equation:
6x = 120 - 60
6x = 60
To solve for x, we divide both sides of the equation by 6:
x = 60 / 6
x = 10
The value of x is 10.
#### 10
The answer is: 10
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
Karen's class fund contains only $10 and $20 bills,  which amount to $120. The number of $x bills is twice as many $20 bills. They have 3 $20 bills in their fund.

### Answer:
