Explanation

Let's say the person's usual travel time to his office is x minutes.

Since he is walking at 5/6 of his usual speed, his new speed is 5/6 of his usual speed.

The time taken to cover the same distance at this new speed is 6/5 of his usual time (since speed and time are inversely proportional).

So, the new time taken = (6/5)x

Since he is 40 minutes late, the new time taken is 40 minutes more than his usual time:

(6/5)x = x + 40

Simplifying the equation:

(1/5)x = 40

x = 200 minutes

Converting x to hours and minutes:

x = 3 hours 20 minutes