To calculate the total change in the amount of water in the tank after 6 days, let's start with the rate of decrease.
The tank decreases by 10 gallons every 3 hours. First, we need to determine how many hours are in 6 days:
\[ 6 \text{ days} \times 24 \text{ hours/day} = 144 \text{ hours} \]
Next, we find out how many 3-hour intervals fit in 144 hours:
\[ \frac{144 \text{ hours}}{3 \text{ hours/interval}} = 48 \text{ intervals} \]
Now, we multiply the number of intervals by the decrease per interval:
\[ 48 \text{ intervals} \times 10 \text{ gallons/interval} = 480 \text{ gallons} \]
Since the tank is decreasing in water, the total change is a negative amount. Therefore, the total change in the amount of water in the tank after 6 days is:
\[ -480 \text{ gallons} \]
So the correct answer is d) -480 gallons.