Question 115451
Let Anna has x number of $5 bills and y number of $10 bills.
Totally, there are 12 bills. 
Therefore, x + y = 12---------(1).
The  value of $5 bills = 5*x = 5x.
The value of $10 bills = 10*y = 10y
So, the total value of the bills = 5x + 10y.
It is given that the total value of the bills = $100.
Therefore, 5x + 10y = 100.
That is, x + 2y = 20-----------(2).


Solve (1) and (2).

(1)*2 gives, 2x + 2y = 24------(3)
(2) gives     x + 2y = 20-------(2)

(3) - (2)    x = 4
Plug in x = 4 in (1), we get y = 8.

So, Anna has 4 bills of $5 and 8 bills of $10