Question 1204798
This helps make clear what is going on with the described information:
<pre>
CLASS        COUNT STUDENTS         PASS        FAIL

A                a                (3/4)a        (1/4)a

B                b                (5/8)b        (3/8)b

TOTAL                               84
</pre>
The number of fails of each class was given as they were equal.


{{{system((3/4)a+(5/8)b=84,a/4=3b/8)}}}
Solve this system in whatever way you want.


{{{system(6a+5b=672,a=3b(1/2))}}}


{{{6(3b/2)+5b=672}}}


{{{9b+5b=672}}}


{{{14b=672}}}


{{{highlight(b=48)}}}

from that, 
{{{highlight(a=72)}}}.