Ответ:

1. 7/15 - 1/6) + 2/5 = 7/10.

2. (3/8 - 1/9) + 25/36 = 23/24.

3. (8/9 - 5/6) + 2/3 = 13/18.

4. (7/8 - 13/20) + 9/10 = 1  1/8.

Пошаговое объяснение:

1. (7/15 - 1/6) + 2/5;

1)  7/15 - 1/6 = 14-5/30 = 9/30 = 3/10

2) 3/10 + 2/5 = 3/10 + 4/10 = 7/10

(7/15 - 1/6) + 2/5 = 7/10.

2. (3/8 - 1/9) + 25/36;

1) 3/8 - 1/9 = 27/72 - 8/72 = 19/72

2) 19/72 + 25/36 = 19/72 + 50/72 = 69/72 = 23/24

(3/8 - 1/9) + 25/36 = 23/24.

3. (8/9 - 5/6) + 2/3;

1) 8/9 - 5/6 = 16/18 - 15/18 = 1/18

2) 1/18 + 2/3 = 1/18 + 12/18 = 13/18

(8/9 - 5/6) + 2/3 = 13/18.

4. (7/8 - 13/20) + 9/10

1) 7/8 - 13/20 = 35/40 - 26/40 = 9/40

2) 9/40 + 9/10 = 9/40 + 36/40 = 45/40 = 1  5/40 = 1  1/8

(7/8 - 13/20) + 9/10 = 1  1/8.