To find the percent decrease in water, we can use the formula for percent change:
\[ \text{Percent Decrease} = \left(\frac{\text{Initial Amount} - \text{Final Amount}}{\text{Initial Amount}}\right) \times 100 \]
In this case, the initial amount of water in the tank is 500 liters, and the final amount is 400 liters.
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Calculate the decrease in the amount of water: \[ \text{Decrease} = \text{Initial Amount} - \text{Final Amount} = 500 - 400 = 100 \text{ liters} \]
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Now, calculate the percent decrease: \[ \text{Percent Decrease} = \left(\frac{100}{500}\right) \times 100 = 0.2 \times 100 = 20% \]
Thus, the percent decrease in water is 20%.
The answer is B.