First, we need to calculate the value inside the parentheses:

25 + 1 5/6 = 25 + 1 + 5/6
25 + 1 = 26
26 + 5/6 = 26 + 0.83 = 26.83

Now we substitute this value back into the equation:

(2, 26.83)6/7 - 0.5

First, we need to multiply 26.83 by 6 and then divide by 7:

(2, 26.83)6/7 = 26.83 * 6 / 7
= 161 / 7
= 23

Now, we subtract 0.5 from 23:

23 - 0.5 = 22.5

Therefore, the final answer is 22.5.