The speed of a boat is 12 km/h in still water. It goes 18 km downstream and comes back in a total time of $6\frac{6}{7}$ hours. Find the speed of the stream (in km/h)
Answer & explanation
Correct answer: option 4
Speed of boat = 20 km/h
Speed of stream = S km/h
Downstream speed = ( 12 + S) km/h
Upstream speed = (12 - S) km/h
According to question ,
\(\frac{18}{12-S}\) + \(\frac{18}{12 + S}\) = \(\frac{48}{7}\) hours
\(\frac{18 × 12 + 18× 12 }{(12-S)(12+S)}\) = \(\frac{48}{7}\)
432 × 7 = 48 × ( 144 - S2 )
on solving ,
S2 = 81
S = 9 km/h
SO , Speed of stream is 9 km/h