He has covered a distance of 5 km from the railway station to his house.
Time taken to cover 1 km at a speed of 20 km/h is 3 min.
So, total time taken for travelling 5 km is 3×5=15 min.
Time spent in chatting with his friend's sister is 10 min.
Total time taken from the railway station to his house is 15 + 10 = 25 min.