There are 21 bills in total, so you have a:
6/21 chance of drawing single
9/21 of drawing five
3/21 of drawing twenties
3/21 of drawing hundred dollar
Fair price:
1 x 6 + 5 x 9 + 20 x 3 + 100 x 3 = 411
411/21 = $19.5714
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