а) 11/30 + 7/12 = 22/60 + 35/60 = 57/60 = 19/20
б) 1/27 + 5/18 = 2/54 + 15/54 = 17/54
в) 5/21 - 5/28 = 20/84 - 15/84 = 5/84
г) 11/18 - 7/12 = 22/36 - 21/36 = 1/36