Ответ:

1. 2 31/90

2. 1 7/48

3.  337/450

Пошаговое объяснение:

1. 5/6 - 1/15 + 7/9 + 4/5 = 75/90 - 6/90 + 70/90 + 72/90 =

  217/90 - 6/90 = 211/90 = 2 31/90

2. 1/12 + 7/16 - 1/8 + 3/4 = 4/48 + 21/48 - 6/48 + 36/48 =

   61/48 - 6/48 = 55/48 = 1 7/48

3. 4/25 + 7/15 - 7/9 + 9/10 = 72/450 + 210/450 - 350/450 + 405/450 =

   687/450 - 350/450 = 337/450