To solve this problem, let's assume the person's usual time is represented by "x" minutes.

According to the given information, the person is walking at 5/6 of his usual rate. This means he is walking at a speed of (5/6)x minutes per minute.

We are also given that the person is 40 minutes late. This means the actual time taken by the person is (x + 40) minutes.

We can set up an equation based on the given information:

(x + 40) = (5/6)x

To solve for x, we can multiply both sides of the equation by 6 to eliminate the fraction:

6(x + 40) = 5x

Expanding the equation:

6x + 240 = 5x

Now, we can subtract 5x from both sides:

x + 240 = 0

x = -240

Since time cannot be negative, we can discard the negative solution.

Therefore, the person's usual time is 240 minutes, which is equivalent to 4 hours.

Converting 240 minutes to hours and minutes:

240 minutes = 4 hours

So, the person's usual time is 4 hours.

The correct answer is C) 3 hours 20 minutes.