Last year, the number of students in Oscar's class was 20% less than the number of students in Haoxi's class. This year, the enrollment in Oscar's class dropped by 25% and the enrollment in Haoxi's class dropped by 10%. Oscar now has 6 fewer students than Haoxi. How many students were in Haoxi's class last year?

Let
x----------> students in Oscar's class last year
y----------> students in Haoxi's class last year

we know that
Last year
the number of students in Oscar's class was 20% less than the number of students in Haoxi's class. 
so
x=0.80y-----> equation 1

this year
0.75x=0.90y-6------> equation 2

using a graph tool
see the attached figure

the solution is 
x=16----> students in Oscar's class last year
y=20----> students in Haoxi's class last year

this year
 students in Oscar's class this year------> 0.75*16----> 12  students
 students in Haoxi's class this year------> 0.90*20----> 18   students


the answer is
 students in Haoxi's class last year were 20