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                              Q:
                           
                           
                                                      | A) 4 mile/hr | B) 6 mile/hr | 
| C) 2 mile/hr | D) 5 mile/hr | 
                              Answer:   B) 6 mile/hr
                              
                                                            
                              
Explanation:
                               
                           
                           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 => 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 => 6 mile/hr.
                            
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