To find out how much the amount of water in the canteen should change every 10 minutes, we first need to determine how much water should be consumed in that time frame.
Since hikers are recommended to drink 24 ounces of water per hour, we can break this down into a 10-minute interval. There are 60 minutes in an hour, so:
\[ \text{Water consumed in 10 minutes} = \frac{24 \text{ ounces}}{60 \text{ minutes}} \times 10 \text{ minutes} = 4 \text{ ounces} \]
Thus, every 10 minutes, the amount of water in the canteen should decrease by 4 ounces.
The answer is: −4 ounces.