So for the part of the journey described it takes 9mins. We can calculate what fraction this is of the whole journey as we are told it takes 9mins to get from of the way to halfway so this is:
If it takes 9mins to travel of the journey takes 3mins. That then means that the whole journey takes 30mins.
If he is halfway (15mins) at 16.05 then he must arrive 15mins later at 16.20, meaning he left at 15.50.