Given :
Speed of upstream = 12 km/hr
Speed of flow of river = 6 km/hr
Formula Used
Time = Distance/Speed
Downstram Speed = Speed of sailer + Speed of water
Upstream Speed = Speed of sailer - Speed of water
Calculation Let the distance from one side be D
According to the formula used
12 = Speed of sailer - 6
Speed of sailer = 18 km/hr
So, The speed in Downstream = 18 + 6 = 24 km/hr
According to the question
()+()=1.25hr ⇒
()=⇒ D = 10 km
So, Total distance = 10 + 10 = 20 km