Начнем с действий в скобках:

1) 1 2/3 + 5/6 = 5/3 + 5/6 = 10/6 + 5/6 = 15/6 = 5/2 = 2.5;

2) 1 - 2/5 = 5/5 - 2/5 = 3/5 = 0.6.

3) 2.5 * 0.6 = 1.5.