Acollege writing seminar increased its size by 50%, percent from the first to the second day. if the total number of students in the seminar on the second day was 15, how many students were in the class on the first day?

The number of students in the class on the first day was 10.

Let x be the number of students in class on the first day.  50% = 50/100 = 0.5; adding 50% more the second day can be represented by:

x+0.5x = 15
1.5x = 15

Divide both sides by 1.5:
1.5x/1.5 = 15/1.5
x = 10

There were 10 students the first day.