Because the units(minutes) for both 45 and 60 are the same, you can simply place 45 over 60 ~ 45min/60min = 3/4 (As the minutes cancel)
It is probably so that one can measure time better. Instead of 1/10 of an hour, one can say 6 minutes.
20%
36 / 60 = 0.6 = 60 %
Check out SCintific American they have a very good article on this.