Assuming the problem includes something like "all of the reamining students eat pears" and "no student eats more than one fruit" then:
1/2 + 1/8 + 1/3
12/24 + 3/24 + 8/24 = 23/24
Out of everyone (24/24) that means 1/24 are left.
(1/24) * 89 = 3.7083333
3 students. Or 4 students?
1/2 eat grapes, so 89 * 1/2 = 44.5
1/8 eat apples, so 89 * (1/8) = 11.125
1/3 eat bananas, so 89 * 1/3 = 29.6666
total: 85.29166666
89 - 85.29166666 = 3.70833333
Huh.
The exact answer is 3.70833333 (infinite 3s) students, but you can't have a fractional student, so the answer is either "3 or 4 students" or "no solution."