а) 7/18 < 5/12, так как 14/36 < 15/36</p>
б) 11/36 > 13/48, так как 44/144 > 39/144
3/8 + 5/12 = 9/24 + 10/24 = 19/24
7/15 - 3/20 = 28/60 - 9/60 = 19/60
23/40 - 9/20 + 1/16 = 46/80 - 36/80 + 5/80 = 15/80 = 3/16
7/13 - а = 7/26 4,72с + 2,8с = 78,96
а = 7/13 - 7/26 7,52с = 78,96
а = 14/26 - 7/26 с = 78,96 : 7,52
а = 7/26 с = 10,5