A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is:
Let the speed of the stream be x km/hr
Speed downstream =(15+x)km/hr
Speed upstream =(15−x)km/hr
Distance travelled downstream=30km
Distance travelled upstream=30km
Time upstream=Distance upstreamSpeed upstream
Time upstream=3015−x
Time downstream=Distance downstreamSpeed downstream
Time downstream=3015−x
Total time taken 4.5hrs
4.5=3015−x+3015+x
4.5=30(15+x)+30(15−x)225−x2
4.5=900225−x2
225−x2=200
x2=25
x=5km/hr
Hence the correct option is (B).