if it took an average man 15 mins to walk 1 mile , then he stopped for a break for 30 seconds then walked half a mile back then stopped for a further 30 seconds and returned to the end point of his original journey , how long has it took him to walk the half mile back ?
As folk have mentioned, there's no data on the speed of that last part. One can only make the assumption that it was the same speed as he initially walked.
you have to calculate his original half mile , part 1 of the journey then remember how long he stopped for then the time he took to return to the middle and then back to the end