Let x be the number of $10 bills.
So, there are (x + 1) $20 bills.
Adding the $10 and $20 bills should sum up to $80 so the equation is 10x + 20(x + 1) = 80.
Then, distribute 20 to x + 1 so it becomes 10x + 20x + 20 = 80.
Combining the like terms on the left side gives 30x + 20 = 80.
Then transfer 20 to the right side of the equation, so it becomes 30x = 60.
To solve for x, we have 60/30 = <<60/30=2>>2.
Therefore, there are 2 $10 bills.
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