28/40+10/75 = 7/10 + 2/15 = 21/30 + 4/30 = 25/30 = 5/6
_______________________________________________
12/27+14/21 = 4/9 + 2/3 = 4/9 + 6/9 = 10/9 = 1 1/9
_______________________________________________
12/18+5/60 = 2/3 + 1/12 = 8/12 +1/12 = 9/12 = 3/4
_______________________________________________
4/24+3/36 = 1/6 + 1/12 = 2/12 + 1/12 = 3/12 = 1/4