1)

15 - 7 2/3  = 14 3/3 - 7 2/3 = 7 1/3,

2)

14 5/7 + 11  = 25 5/7,

3)

8 * (-3 1/4)  = 8 * (-13/4) = -2 * 13 = -26,

4)

3 3/4 : 5/8 = 15/4 * 8/5 = 3 * 2 = 6