(-24/55) / (39/40)
(-24/55) * (40/39)
(-24 * 40) / (55 * 39)
cancel a 3 from the 24 and 39:
(-8 * 40) / (55 * 13)
cancel a 5 from the 40 and 55:
(-8 * 8) / (11 * 13)
11 and 13 are prime and there are no 11s or 13s on top, so we can't reduce any further
-64 / 143