If

of students were juniors and seniors that leaves the 36 sophomores to be

or

of attendees. I got

by converting

and adding it to

.
Now if 36 is

of the equation that means theirs at least 36 juniors and seniors.
Lets start with seniors. there should be just over 36 of them there. Lets start by multiplying 36 by

to figure out how many

is.
The equation would look like this 36*(3/12)
start with 3/12. this equals .25 or

.
Now multiply 36 by .25. this equals 9.
We have 9 students per

of the attendees.
To get the amount of seniors add 9 to 36

because we already stated

is 36.
36+9=45
Now to find the juniors add another 9 to create

.
45+9=54
In total you have:
36 sophomores
54 juniors
45 seniors