To reduce fractions, simply divide both the numerator and the denominator by the GCD of the numerator and the denominator.
For example, to reduce 18/42, you divide the numerator and denominator by 6, since 6 is the GCD of 18 and 42, to get 3/7. However, a fraction such as 7/17 is already reduced entirely, because the GCD of 7 and 17 is 1.
Hope I helped! Feel free to respond to my answer if anything is unclear.
