Answer:
[tex]\large \boxed{\text{17 \$20 bills and 23 \$5 bills}}[/tex]
Step-by-step explanation:
Let x = the number of $20 bills.
Then x + 6 = the number of $5 bills and
20x + 5(x + 6) = 455
[tex]\begin{array}{rcl}20x + 5(x + 6) & = & 455\\20x + 5x + 30 & = & 455\\25x + 30 & = & 455\\25x & = & 425\\x & = &\dfrac{425}{25}\\\\x & = &\mathbf{17}\\\end{array}[/tex]
x = 17 = number of $20 bills
x + 6 = 17 + 6 = 23 = number of $5 bills
[tex]\text{There were }\large \boxed{\textbf{17 \$20 bills and 23 \$5 bills}}[/tex]
Check:
[tex]\begin{array}{rcl}20\times17+ 5(17 + 6) & = & 455\\340 + 5\times23& = & 455\\340 + 115 & = & 455\\455 & = & 455\\\end{array}[/tex]
OK.