To find the percentage decrease in the depth of the lake from January to August, we can use the following formula for percentage decrease:
\[ \text{Percentage Decrease} = \frac{\text{Original Value} - \text{New Value}}{\text{Original Value}} \times 100 \]
In this case, the original value (depth in January) is 873 feet, and the new value (depth in August) is 698.4 feet.
First, we calculate the decrease in depth:
\[ \text{Decrease} = 873 - 698.4 = 174.6 \text{ feet} \]
Next, we plug the values into the percentage decrease formula:
\[ \text{Percentage Decrease} = \frac{174.6}{873} \times 100 \]
Calculating the fraction:
\[ \frac{174.6}{873} \approx 0.2005 \]
Now multiply by 100 to convert it to a percentage:
\[ 0.2005 \times 100 \approx 20.05% \]
Therefore, the percentage decrease in the depth of the lake from January to August is approximately 20.05%.