If S is the number of students 0.9S-36=34, so 0.9S=70 and S=70/0.9=77.8!
Let’s round this up to 78, so 90% of 78 is about 70. 36 students leave by 5pm, leaving 34.
So we have to assume that 90% is only an approximate percentage. 70/78 is about 89.74%.