Let x be the number of students in Emily's class such that 75% of x is 15.
15 = 0.75x
The value of x is equal to 20.
Let y be the number of students in Santiago's class. Similar with what we did previously,
12 = (2/3)y
The value of y is equal to 18.
Therefore, there are more student's in Emily's class than in Santiago's class.