7 - 15/12 = 6 12/12 - 1 3/12 = 5 9/12 = 5 3/4
5 3/4 : 6,7 = 5 3/4 : 6 7/10 = 23/4 * 10/67 = 23/2 * 5/67 = 115/134
5,75 - 31/6 = 5 75/100 - 5 1/6 = 5 225/300 - 5 50/300 = 175/300 = 7/12
7/12 : 15,5 = 7/12 : 155/10 = 7/12 * 10/155 = 7/6 * 5/155 = 35/930
115/134 + 35/930 = (115*930)/124620 + (35*134)/124620 = 106950/124620 + 4690/124620 = 111640/124620 сокращаем на 20 = 5582/6231