A man swimming in a stream which flows 1 1/2 km/hr finds that in a given time he can swim twice as far with the stream as he can against it. At what rate does he swim?
ক) 4 1/2 km/hr
খ) 5 1/2 km/hr
গ) 7 1/2 km/hr
ঘ) 3 1/2 km/hr
