Question

A runner runs 90 meters with the wind and 70 meters against the wind in 10 seconds. How long does he need to run 100 meters without wind?

Answer 1

`12.5 s`

Explanation:

Let,the velocity of the runner is x m/s and that of wind is u m/s

So,when moving with the wind,his velocity becomes `(x+u) m/s`,so if he goes `90 m` in `10 s` in that direction,we can say `10=90/(x+u)`

Again,when moving opposite to the wind,his velocity becomes `(x-u) m/s` so if he goes `70 m` in that direction for `10 s`,we can say, `10=70/(x-u)`

Solving both we get, `x= 8m/s`

So,he will take `100/8 s` or, `12.5 s` to run for `100 m` without wind.