Suppose in normal days man reaches at 6 and lets consider they usually take 1 hour that means they reach at 7 PM.

Now when he reached early at 5 he started walking. For his wife it will take 1 hour to reach but she met him in before reaching station and they reached home at 6:40PM instead of 7 PM(20 minutes early) . Therefore total time taken by his wife is 6:40 PM - 5:00 PM i.e 1 hour 40 minutes .

So she travelled 50 minutes each side the day her husband reaches early. so she started at 5PM and she met her husband at 5 PM + 50 minutes i.e 5:50 PM so her husband walked for 50 minutes as he started walking at 5:PM.

You may calculate with 2 hrs or 3 hrs and u will end up with the same values.