To find the total change in the amount of water in the tank after 6 days, we start by determining the daily decrease in gallons.
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The tank decreases by 10 gallons every 3 hours. In one day (24 hours), the number of 3-hour intervals is: \[ \frac{24 \text{ hours}}{3 \text{ hours}} = 8 \text{ intervals} \]
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Therefore, the daily decrease in water is: \[ 10 \text{ gallons/interval} \times 8 \text{ intervals} = 80 \text{ gallons} \]
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Over 6 days, the total decrease in water is: \[ 80 \text{ gallons/day} \times 6 \text{ days} = 480 \text{ gallons} \]
Since the total change is a decrease, we express it as negative: \(-480\) gallons.
Thus, the correct answer is:
−480 gallons.