Question

A race boat covers a distance of 60 km/ hr in one and half hour. It covers this distance upstream in 2 hrs. The speed of the race boat in still water is 35 km/ hr . Find the speed of the stream

Answer 1

As the speed of the boat in still water is known to us we can calculate the speed of the stream either any stream i.e. down stream or up stream.

Say, the speed of the stream is x km/hr.

Going downstream the speed of the boat and the speed of the stream will be added together i. e. (35+x) km/hr

Hence the boat goes (35+x) km in 1 hr.

For a distance of 60 km, the Time =6035+xhr

so, 6035+x=112=32

or, 3(35+x)=2⋅60 [cross multiplication]

or, 105+3x=120

or 3x=120−105

or, x=153=5

Speed of the stream is 5 km/hr.

Another way
Going upstream, the speed of the stream is deducted from speed of the boat i.e (35−x) km/hr

Hence goes (35−x) km in 1 hr.

Therefore for a distance of 60 km Time = 6035−x hrs.

As per question, Time is 2 hours, so 6035−x=2

or 60=2(35−x) [ cross multiplication]

or, 60=70−2x

or 2x=70−60

or, x=102=5

So, speed of the stream is 5 km/hr.