Question

Aboat covers 120 km downstream in 8 hours and the same distance covers in upstream in
40 hours. If the speed of boat in still water and speed of stream is increased by 6 kmph
and 4 kmph respectively, then what is the total time taken by the boat to covers 200 km in
upstream and downstream?​

Answer 1

Answer:

Upstream = 40 hours

Downstream = 8 hours

Step-by-step explanation:

Suppose the speed of boat in still water = x.

and the speed of stream = y.

In upstream, speed = x - y.

In downstream, speed = x + y.

Time = Distance / Speed

$\frac{120}{x + y} = 8 \\ \frac{15}{x + y} = 1 \\ x + y = 15 \: \: \: ...(1)$

$\frac{120}{x - y} = 40 \\ \frac{3}{x - y} = 1 \\ x - y = 3 \: \: \: ...(2)$

Adding (1) and (2), 2x = 18 so x = 9.

And we get, y = 6.

New speed of boat = 15 kmph

New speed of stream = 10 kmph.

New upstream speed = 5 kmph.

New downstream speed = 25 kmph.

Time for upstream = Distance / Speed = 200/5 = 40 hours.

Time for downstream = Distance / Speed = 200/25 = 8 hours.

Took me long time to answer, really hope this helps.