To determine how much the amount of water in the canteen should change every 10 minutes, we start with the recommendation of 24 ounces of water per hour.
Since 1 hour equals 60 minutes, we can calculate the amount of water that should be consumed every 10 minutes:
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Calculate the amount of water recommended per minute: \[ \text{Water per minute} = \frac{24 \text{ ounces}}{60 \text{ minutes}} = 0.4 \text{ ounces per minute} \]
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Now, multiply this by the number of minutes (10 minutes): \[ \text{Water for 10 minutes} = 0.4 \text{ ounces per minute} \times 10 \text{ minutes} = 4 \text{ ounces} \]
Since this is the amount of water consumed every 10 minutes, the water in the canteen should decrease by 4 ounces.
Thus, the correct response is: −4 ounces (negative 4 ounces).