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? Think carefully first, then make a decision:
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.