Question

Speed of a boat in still water is 15 km/h. It goes 30 km upstream and returns

back at the same point in 4 hours 30 minutes. Find the speed of the stream​

Answer 1

Answer:

A boat whose speed in 15 km/hr in still water goes 30 km downstream and come back in a total of 4 hours 30 minutes. What is the speed of the stream (in km/hr)?

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I’m somewhat surprised that a physics question is talking about speed instead of velocity!

Hopefully, you know that the distance covered by an object moving at constant speed is simply its speed multiplied by the time taken, thus s=vt . Dividing both sides by v , we have:

t=sv

Let’s first consider the movement downstream. The distance covered is simply 30 km. Using u as the speed of the stream, the downstream speed is 15+u km/hr.

Using our eq

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Thanks to Alpana Sachan for this question, because this is an awesome question

Let's assume that the speed of stream is = x km/h

Speed of boat in still water v = 15 km/h

Total time taken = 4 hour and 30 mins or 9/5 hrs

Distance covered (one way) = 30 kms

Then,

Speed of boat downstream =( 15 + x ) km/h

Time taken downstream = (30/15 + x ) hrs

As time = distance / speed

Speed of boat upstream = ( 15 - x ) km/h

Time taken upstream =( 30/15 - x ) hrs

Now,

Total time taken = Time taken downstream + Time taken upstream

9/2 = (30/15 + x ) + (30/15 - x )

Skipping the calculation part ( I was gonna type it out but i realized it'll take a LOT of time + you guys are smart enough to do that)

x = 5 km/h

The speed of the stream is 5 km/h

I hope my answer helps...

Answer 2

$\huge\underline\mathfrak\purple{SolutiOn:}$

Let the of the stream be x km/h.

Time taken to cover 30 km upstream = 30/15 - x

Time taken to cover 30 km downstream = 30/15 + x

According to the Question,

⇒ 30/(15 - x) + 30/(15 + x) = 9/2

⇒ 30[(15 - x) + (15 - x)/(15 - x)(15 + x)] = 9/2

⇒ 30[30/225 - x²] = 9/2

⇒ 225 - x² = 200

⇒ x² = 225 - 200

⇒ x² = 25

⇒ x = - 5, 5 (As speed can't be negative)

⇒ x = 5

Speed of stream = x = 5 km/h.

Hence, the speed of stream is 5 km/h.