You have 7 ones
3 fives
2 tens
6 twenties
This is a total of 7+3+2+6 = 18 bills
First we want to know the probabilitity that you will draw a five
P(five dollar bill) = number of fives/ total = 3 / 18 = 1/6
Now we have 7 ones
2 fives
2 tens
6 twenties
This is a total of 7+2+2+6 = 17 bills left ( assuming you get to keep the bill)
Now we want to know the probability of getting a twenty dollar bill
P( twenty dollar bill) = number of twenties / total = 6/ 17
Multiply the probabilities together
1/6 * 6/17 = 1/17
The probability of getting a five and then a twenty is 1/17