If it's not repeating:

rewrite the decimal number as a fraction (over 1), then multiply the numerator and the denominator by 10^{y} (y being the number of decimal places it has), then simplify

FOR EXAMPLE:

0.325/1=325/1000=13/40

If it's repeating:

name the decimal x, then multiply it by ten until it repeats the second time, the number should be 10^{y}x (y being the number of numbers before it repeats the second time).

Subtract 10^{y}x by x and simplify until you get a fraction.

FOR EXAMPLE:

0.654654654654...=x

0.654654654654... it keeps repeating "654" so we have to multiply it by 10^{3}.

654.654654654654...=1000x

1000x-x=654.654654654...- 0.654654654654=

999x=654

x=654/999

x=218/333