а) (3/4 + 7/12) 4 = (9/12 + 7/12) 4 = (16/12) * 4 = 64/12 = 16/3

б) (5/6 - 1/18) / 3 = (15/18 - 1/18) / 3 = 14/18 / 3 = 14/18 * 1/3 = 14/54 = 7/27