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Speed of man $(m) = 8\;km/h$
Let the speed of stream be $s$

According to the question:
Speed of man upstream $= S_1 = m-s$
Speed of man downstream $= S_2 = m+s$

Speed = Distance/Time
Here, since the distance $D$ is same,

$D = S_1 \times T_1 = S_2 \times T_2$

$S_1 \times T_1 = S_2 \times T_2$

$\implies \frac{S_1}{S_2} = \frac{T_2}{T_1} = 1/3$

$m+s = 3(m-s)$

or, $8+s=3(8-s)$

$\implies s=\; 4\;km/h$
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