Explanation:
4 1/2 = 9/2
so, the calculation is simply how often 3/4 fits into 9/2.
and that is done by division :
9/2 / 3/4
and as we know, to divide by a fraction is the same as to multiply with the upside-down fraction.
so,
9/2 / 3/4 = 9/2 × 4/3 = (9×4)/(2×3) = 3×2/1 = 6
so, without any breaks the teacher can schedule 6 lessons each night.