Question

how to solve this question

Answer 1

Let speed of stream be x
Then speed of boat while going downstream will be 15+x (coz the speed of stream works along the direction of the speed of the boat)
Speed of boat while going upstream will be 15-x (coz here the stream's speed works against the boat's speed)
Distance covered downstream and upstream is equal.
Speed=Distance/Time
Time=Speed/Distance
$4 \frac{1}{2} = \frac{30 }{15 + x} + \frac{30}{15 - x} \\ \frac{9}{2} = 30( \frac{1}{15 + x} + \frac{1}{15 - x} ) \\ \frac{3}{20} = \frac{30}{225 - {x}^{2} } \\ 675 - 3 {x}^{2} = 600 \\ 3 {x}^{2} = 75 \\ {x}^{2} = 25 \\ x = + 5 \: or \: - 5 \\ speed \: cannot \: be \: - ve \\ x = 5 \: km \: per \: h$