<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?
<bot>: How many $10 bills are there? ** If n is the number of $20 bills then 2n is the number of $10 bills.
Write an equation ** So the equation will be $10(2n) + $20n = $120.
Combine like terms ** By combining like terms, the equation becomes 40n = 120.
How many $20 bills are there? ** So, the value of n which represents the number of $20 bills is 120/40 = <<120/40=3>>3.
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