The respective ratio between speed of the boat upstream and speed of the boat downstream is 3 : 4. What is the speed of the boat in still water if it covers 70 km downstream in 3 hours 30 minutes? (in km/h)
Let the speed of boat in still water and speed of river be B km/hr and R km/hr respectively
Speed of boat in downstream = (B+R)km/hr
Speed of boat in upstream = (B-R) km/hr
It is given that $$\frac{B-R}{B+R} = \frac{3}{4}$$
we get , B = 7 R
Now it is given that boat covers 70 km in 3.5 hours downstream
so ,
70 = (B+R)3.5
B+R = 20
R = 2.5 km/hr
B = 17.5 km/hr
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