+1667 Not quite sure if you mean in general or on the calculator, but if you mean in general, all you have to do is take the decimal, put it over 1, and multiply by 10 until you get a whole number. Then, simplify.
Ex.
0.255
Put it it over 1...
0.255/1
Multiply by 10 until whole...
255/1000
Simplify...
51/100
