0.83 * 100 = 83, so 0.83 = 83/100.
But in case you meant the slightly more interesting repeating decimal, 0.8333...:
Let x denote this number. Then
10x = 8.333...
100x = 83.333...
==> 100x - 10x = 83.333... - 8.333...
==> 90x = 83 - 8 = 75
==> x = 75/90 = 5/6
