By factoring out the greatest common factor, as shown below, we can find our answer. Realize that 25 "goes in" to both 25 and 75, meaning that we can simplify the fraction.
[tex]\dfrac{25}{75} = \dfrac{25 \cdot 1}{25 \cdot 3} = \dfrac{1}{3}[/tex]
The simplified fraction is 1/3.