To find the water level of the river at the end of summer, we need to add the two drops in water level:
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The initial water level is \( 1 \frac{1}{2} \) meters below sea level. This can be converted to an improper fraction: \[ 1 \frac{1}{2} = \frac{3}{2} \text{ meters} \]
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During the summer, the river drops another \( 2 \frac{1}{3} \) meters below sea level. This can also be converted to an improper fraction: \[ 2 \frac{1}{3} = \frac{7}{3} \text{ meters} \]
Next, we need to add these two fractions together to find the total drop in water level:
\[ \frac{3}{2} + \frac{7}{3} \]
To add these fractions, we need to find a common denominator. The least common multiple of 2 and 3 is 6.
Now let's convert each fraction: \[ \frac{3}{2} = \frac{3 \times 3}{2 \times 3} = \frac{9}{6} \] \[ \frac{7}{3} = \frac{7 \times 2}{3 \times 2} = \frac{14}{6} \]
Now we can add the fractions: \[ \frac{9}{6} + \frac{14}{6} = \frac{9 + 14}{6} = \frac{23}{6} \]
So, the total drop in water level is \( \frac{23}{6} \) meters below sea level.
To express this as a mixed number: \[ \frac{23}{6} = 3 \frac{5}{6} \]
Thus, the water level at the end of summer is \( 3 \frac{5}{6} \) meters below sea level.
So the final answer is: \[ \text{The water level is } 3 \frac{5}{6} \text{ meters below sea level.} \]