A son and father go for boating in river upstream. After rowing for 1 mile son notices the hat of his father's hat is falling in the river. After 5 min., he tells his father that his hat has fallen. So, they turn around and are able to pick the hat at the point from where they began boating after 5 min. Tell the speed of river.

• after falling the hat son and father traveled for 5 min. after realization they came back to the starting point in 5 min. That means there is a 10 min gap between the falling time and picking time. but in the same time the hat had reach to the starting point and hat will travel with speed of the river because the boat was traveling in the upstream.
  the hat travels 1 mile in 10 min or in 1/6 hrs.
  speed of the hat=speed of the river=1/(1/6)=6 mph
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