Question

A man in a speed boat which could only travel at 50km/hr was driving his boat up a river which flowed with a constant seed. As he was driving his spare life jacket fell of the boat, 20 minutes later the man noticed the jacket was gone and turned back to get it. he found it 3km away from where he lost it. how fast was the river flowing in km hr

Answer 1

Let's call the speed of the river "r".

The man's speed relative to the shore is the speed of the boat (50 km/hr) minus the speed of the river (r). So when he lost his life jacket, it was carried by the river at a speed of r km/hr for 20 minutes, or 1/3 of an hour.

Distance the life jacket traveled = Speed x Time

So we have:

3 km = r x (1/3) hr

Multiplying both sides by 3 gives:

9 km/hr = r

Therefore, the river was flowing at a speed of 9 km/hr.

Explanation: