In these problems, it is always a good idea to find the total population. In this case, it would be the total number of people who went on the field trip.
28+18+4=50
There are 50 people total.
To find each fraction, just put the number of students/parents/teachers over 50 and simplify.
Students
28/50=14/25
Parents
18/50=9/25
Teachers
4/50=2/25
Hope this helps!