[tex] \frac{15}{35} /5 \\ \frac{15/5}{35/5} \\ \frac{3}{7} [/tex]
That's the simplest form of 15/35. You can use 3/7 to get other equivalent fractions by multiplying the numerator and denominator by the same number. Other examples are 9/21, 6/14, and 12/28.
Hope this helps!