Question 327037
12 $5, 8 $10, 5 $20=25 bills total
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Remember for the second probability, the total number of bills is reduced by 1.
1.{{{P(5,5)=P(5)*P(5)= (12/25)*(11/24)=11/50}}}
2.{{{P(5,10)=P(5)*P(10)=(12/25)*(8/24)=8/50}}}
3.{{{P(5,20)=P(5)*P(20)=(12/25)*(5/24)=1/10}}}
4.{{{P(10,5)=P(10)*P(5)= (8/25)*(12/24)=8/50}}}
5.{{{P(10,10)=P(10)*P(10)= (8/25)*(7/24)=7/75}}}
6.{{{P(10,20)=P(10)*P(20)= (8/25)*(5/24)=1/15}}}
7.{{{P(20,5)=P(20)*P(5)= (5/25)*(12/24)=1/10}}}
8.{{{P(20,10)=P(20)*P(10)=(5/25)*(8/24)=1/15}}}
9.{{{P(20,20)=P(20)*P(20)=(5/25)*(4/24)=1/30}}}
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If you add all of these probabilities together, you will get {{{1}}}, since these outcomes represent all of the possible outcomes. 
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Now find, the ones that sum to at least $20 and add up their probabilities.
3,5,6,7,8,9 all sum to at least $20. Add up all of their probabilities or you could add 1,2, and 4 probabilities and subtract from 1.
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P(>=$20)={{{23/50}}} or 46%