the paper money are $1, $2, $5, $10 and $20

one way is for the bunk to just simply give you $20

lets see what are the other ways

let

a = the number of $1

b = the number of $2

c = the number of $5

d = the number of $10

and a, b, c, d are non-negative integers such that

a + 2b + 5c + 10d = 20

also we have the following constrains

d<=2

c<=4

b<=10

a<=20

just count the number of solutions of

a + 2b + 5c + 10d = 20

and add 1 (for the $20 bill payment) and you will get your answer.