To find this out you put the number that ride the bus over the amount of students. 21/28.
Then, you simplify it with a GCF. {Greatest Common Factor} The GCF here is 7. 21 divide by 7 is 3. 28 divide by 7 is 4. The simplified fraction is 3/4.
The percentage is based on the simplified fraction. 3/4 translates to 75%. You can figure this out by dividing the numerator by the denominator.
Hope this helps!