You have to assume that the choices are mutually exclusive. That means that if a student likes PE he can't choose Social Studies. If he likes Social Studies best then he doesn't like PE best.
Begin by finding out what 75% of the students are.
75/100 * 20 = 75 * 20/100 = 1500/100 = 15.
Fifteen students prefer PE. Only 4 like Social Studies.