Answer:
5 $20 bills and 7 $10 bills
Step-by-step explanation:
You need to have 12 bills, and their values must add up to $170. Start with a guess, and adjust the guess until you find the answer.
The total number of $20 bills and $10 bills must be 10.
I made a table below. I started with a guess and kept adjusting it.
Number of          Number of          Total
$20 bills               $10 bills               Value
8   ($160)              4    ($40)              $200      Value is too high.
6   ($120)              6    ($60)              $180       Value is close
5   ($100)              7    ($70)               $70        Correct value
Answer: 5 $20 bills and 7 $10 bills