Question 1006041
<pre>
Let the number of $10 bills be x
Let the number of $20 bills be y


                      Value      Value
Type       Number       of         of
 of          of        EACH       ALL
bill        bills      bill      bills
-------------------------------------------
$10 bills     x        $10      $10x
$20 bills     y        $20      $20y
-------------------------------------------
TOTALS       16      -----      $220

 The first equation comes from the second column.

  {{{(matrix(3,1,Number,of,$10 bills))}}}{{{""+""}}}{{{(matrix(3,1,Number,of,$20 bills))}}}{{{""=""}}}{{{(matrix(4,1,total,number,of,bills))}}}

                 x + y = 16

 The second equation comes from the last column.
  {{{(matrix(4,1,Value,of,ALL,$10 bills))}}}{{{""+""}}}{{{(matrix(4,1,Value,of,ALL,$20 bills))}}}{{{""=""}}}{{{(matrix(5,1,Total,value,of,ALL,bills))}}}


             10x + 20y = 220

 So we have the system of equations:
           {{{system(x + y = 16,10x + 20y = 220)}}}.

We solve by substitution.  Solve the first equation for y:

                x + y = 16
                    y = 16 - x

Substitute (16 - x) for y in 10x + 20y = 220

     10x + 20(16 - x) = 220
      10x + 320 - 20x = 220
           -10x + 320 = 220
                -10x = -100
                   x = 10 = the number of $10 bills.

Substitute in y = 16 - x
              y = 16 - (10)
              y = 6 $20 bills.

Checking:  10 $10 bills is $100 and 6 $20 bills is $120
            That's 16 bills.
            And indeed $100 + $120 = $220
Edwin</pre>