Discussion

Ariver 1.5 m deep and 36 m wide is flowing at the rate of 3.5 km per hour. The amount of water that runs into the sea per minute (in cubic metres) is

Answer: B

The water leaving each minute forms a prism whose cross-section is the river's channel and whose length is the distance travelled in that minute.

\[ \text{cross-section} = 1.5 \times 36 = 54 \text{ m}^{2} \]

\[ \text{speed} = 3.5 \text{ km/h} = \frac{3500}{60} = 58.33 \text{ m/min} \]

\[ V = 54 \times 58.33 = 3150 \text{ m}^{3} \text{ per minute} \]

Converting km/h to metres per minute, not per second, is what matches the question's units; per hour the figure would be 189,000 m³.

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