A son and father goes for boating in river upstream . After rowing for 1 mile son notices the hat of his father falling in the river. After 5 min he tells his father that his hat has fallen. So they turn round and are able to pick the hat at the point from where they began boating after 5min. Find the speed of river in miles/hours ?  Select:
A) 7 mile/hr	B) 6 mile/hr	C) 8 mile/hr	D) 1 mile/hr	E) 2 mile/hr
Explanation:   Let the speed of river and boat be 'r' m/min and 'b' m/min.   so relative speed in upstream (b-r)m/min and in downstream (b+r)m/min.   Now in upstream distnce covered in 5 min is 5(b-r)miles   so total distnce covered =&gt; 1 + 5(b-r)miles in upstream   In downstream distance covered in 5min is 5(b+r)miles   Now 1 + 5(b-r) = 5(b+r)   1+5b-5r = 5b+5r   1 = 10r   r = 1/10 mile/min =&gt; 6 mile/hr.   Answer: 6 mile/hr
<result>B</result>
A son and father goes for boating in river upstream . After rowing for 1 mile son notices the hat of his father falling in the river. After 5 min he tells his father that his hat has fallen. So they turn round and are able to pick the hat at the point from where they began boating after 5min. Find the speed of river in miles/hours ?