(a) 3 km/hr
Ratio of upstream speed to downstream speed = 1 : 3
∴ Ratio of times taken to cover a certain distance upstream and downstream = 3 : 1
Given, the man took 20 hours to cover the 45 km distance and back ⇒ He took 15 hours to cover 45 km upstream and 5 hours to the cover 45 km downstream.
∴ Speed downstream = \(\frac{45}{5}=9\) km/hr
Speed upstream = \(\frac{45}{15} = 3\) km/hr
∴ Speed of stream = \(\frac12(9-3)\) km/hr = 3 km/hr.