Question 756309
$10 bills = x
$5  bills = y

  x +  y = 125.....1
10x + 5y = 1000....2
Multiply (1) by 5

 5x + 5y = 625.....3
10x + 5y = 1000....2

Subtract (3) from (2)

 5x      =  375
  x      =   75
Substitute x = 75 into Eq (1)

   x + y = 125
  75 + y = 125
       y = 125 - 75
       y = 50
Therefore the teller has:
75 $10 bills and 50 $5 bills.
Proof
75*$10 + 50*$5 = $1000
Hope this helps
:-)