The ratio of upstream to downstream speed is U : 3 respectively. If the speed of the stream doubles then the boat takes 'U' hours to cover a distance of 70 km downstream if the still water speed is (U + 3)2 km/hr and the value of 'U' is an 'integer' then find the time taken by boat to travel 150 km in upstream?
1
2.5 hours
2
5 hours
3
7.5 hours
4
10 hours
5
12.5 hours