Discussion

Explanation:

Let the speed of the boat be Vb and that of the stream be V_w.

Now, the time taken by the boat in going upstream = $\frac{10}{V_b-V_w}$ = 2

⇒ V_b - V_w = 5 ... (i)

And the time taken by the boat in going downstream = $\frac{10}{\frac{V_b}{2}+V_w}$ = 2

⇒ V_b/2 + V_w = 5 ... (ii) 

Adding (i) and (ii),

(3V_b)/2 = 10

V_b = 20/3 

V_w = 5 - (10/3) = 5/3 kmph

The speed of the water flow is 5/3 kmph.

Hence, option (a).

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