Let the tens be x.
Therefore the hundreds will be (26 - x).
Since they both total to 26. x + (26 - x) = 26
10*x + 100*(26 - x) = 1610
10x + 2600 - 100x = 1610
10x - 100x = 1610 - 2600
-90x = -990
x = -990/-90
x = 11.
So the 10s are x =11, the 100s are (26 -11) = 15
11 tens bills and 15 hundred bills.