To express the total gallons of water used in the water park after adding the new ride, we will first express both quantities in scientific notation.
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The current amount of water used in the park is \( 2.1 \times 10^7 \) gallons.
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The additional water used by the new ride is \( 680,000 \) gallons. To express this in scientific notation:
\[ 680,000 = 6.8 \times 10^5 \]
Now, we need to add \( 2.1 \times 10^7 \) and \( 6.8 \times 10^5 \). Since these numbers are in different orders of magnitude, we need to adjust them to have the same exponent for easier addition:
Convert \( 6.8 \times 10^5 \) to the same order of magnitude as \( 2.1 \times 10^7 \):
\[ 6.8 \times 10^5 = 0.068 \times 10^7 \]
Now we can add:
\[ 2.1 \times 10^7 + 0.068 \times 10^7 = (2.1 + 0.068) \times 10^7 = 2.168 \times 10^7 \]
Thus, the total gallons of water used in the park after installing the new ride is:
\[ \boxed{2.168 \times 10^7} \]