# A vector problem

I couldn't solve this problem.Can anyone help??

A man can cross a 100 meter width river in 4 minutes straightly when there is no current in the river.But when the current is available he can cross it in 5 minutes.What is the speed of the current?

Note by Irtesam Mahmud Khan
4 years, 3 months ago

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His velocity is 100 meters in 4 minutes, or 25 meters per minute.

It states that with a river current, he takes 5 minutes to cross. This means he traveled a total of 125 meters relative to the current.

Now suppose that instead of the river having a current and the man traveling in a straight line, the river is still stationary and the man is traveling in a slanted line. We have that the slanted line is 125 meters, the distance from coast to coast is 100 meters; therefore, he traveled $$\sqrt{125^2-100^2}=75$$ meters away from the point where he would have landed if he had traveled in a line perpendicular to the river. This 75 meters accounts for the current: the current travels 75 meters in 5 seconds, or $$\boxed{25\text{ meters per second}}$$.

- 4 years, 3 months ago

75 meters in five minutes so its 0.25 meters per second.........??

- 4 years, 3 months ago

The last computation is the most awful one, no? "75 minutes in 5 minutes, or 15 meters per minute" (or 0.25 meters per second).

- 4 years, 3 months ago

Thanks

- 4 years, 3 months ago

Its 15m/s.

- 4 years, 3 months ago