Hi,
So if I read correctly, there are 788 students each getting one of four colors of t-shirts (and there are an equal number of shirts per color). To do this, all you need to do is divide the number of students (788) by the number of colors (4) to see how many students get each color of t-shirt.
788 รท 4 = 197
Red is one of the four colors, and 197 is exactly one fourth of the students, so one-fourth of the students (197) get red.