Question

A swimmer jumps from a bridge over a canal and swims $1\text{km}$ upstream. After that first km, he passes a floating cork. He continues swimming for half an hour and then turns around and swims back to the bridge. The swimmer and the cork reach the bridge at the same time. The swimmer has been swimming at a constant speed. How fast does the water in the canal flow ?

• A. $2\text{km/h}$
• B. $3\text{km/h}$
• C. $1\text{km/h}$
• D. $4\text{km/h}$

Answer 1

The correct option is C $1\text{km/h}$

Speed of the swimmer $=v$
Speed of water in canal $=u$

Speed of swimmer upstream $=v-u$
Speed of swimmer downstream $=v+u$

Speed of cork $=u$
When swimmer came across the cork, swimmer's relative speed is $(v-u+u)=v$
So, after $\frac{1}{2}hr$ distance between them = $\frac{v}{2}$
While returning back, swimmers relative speed is $(v+u-u)=v$
Hence time taken by the swimmer to catch the cork $=\frac{v/2}{v}=\frac{1}{2}hr$

Cork traveled $1\text{km}$ in $(1/2+1/2)=1\text{hr}$

Speed of water in canal $=1\text{kmph}$