Let

x = the number of ones,

y = the number of fives, and

z = the number of twenties

A cashier has a total of 30 bills, made up of ones, fives, and twenties:

x + y + z = 30

The number of twenties is 9 more than the number of ones:

z = 9 + x

The total value of the money is $351:

x + 5y + 20z = 351

by solving the system of equations:

x + y + z = 30

z = 9 + x

x + 5y + 20z = 351

we find:

x = 6

y = 9

z = 15

click here to see the step by step solution of the system of equations:

there 6 one dollar bills, 9 five dollar bills and 15 twenty dollar bills.