Let's define the next variable
x: the amount of money each student has to raise
If 1 student raises x dollars, then 15 students will raise 15x dollars.
They want to raise $284.20 and they have already raised $58.75, therefore they need:
$284.20 - $58.75 = $225.45
Combining this information, we get:

Each student has to raise $15.03