Discussion

A man swims downstream 72 km and upstream 45 km taking 9 hours each time; what is the speed of the current?

Answer: B

With \( b \) the still-water speed and \( r \) the stream's,

\[ \text{downstream} = b + r, \qquad \text{upstream} = b - r \]

\[ b + r = \frac{72}{9} = 8 \text{ kmph}, \qquad b - r = \frac{45}{9} = 5 \text{ kmph} \]

\[ r = \frac{8 - 5}{2} = 1.5 \text{ kmph} \]

The 3 kmph option is the full difference; halving it is the step that separates the current from the gap between the two speeds.

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