3. 2,75 - (5/9 + 7/15) : 1 8/15 - 1

2,75 - (5/9 + 7/15) : 1 8/15 - 1 = 2,75 - (25/45 + 21/45) : 23/15 - 1 = 2,75 - 46/45 : 23/15 - 1 = 2,75 - 46/45 * 15/23 - 1 = 2,75 - (46/23) * (15/45) - 1 = 2,75 - 2 * 1/3 - 1 = 2,75 - 2/3 - 1 = 1,75 - 2/3 = 7/4 - 2/3 = 21/12 - 8/12 = 13/12