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?
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 = <<120/40=3>>3.
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