The water aciditiy in a pond is considered normal when the average pH reading of three daily measurement is between 7.1 and 7.8. If the first two pH readings are 7.25 and 7.8,find the range of pH of the water of the pond for the 3rd reading that will result in the acidity level being normal.

Explanation

To find the range of the pH for the third reading that will keep the average pH of the pond water within the normal acidity levels, we can follow these steps:

### Step-by-Step Solution:

1. **Define the average pH condition**:
The average pH of the three readings must be between 7.1 and 7.8. This can be expressed as:
\[ 7.1 < \text{Average} < 7.8 \]

2. **Express the average in terms of the third reading**:
Let the third pH reading be \( X \). The average of the three readings (7.25, 7.8, and \( X \)) can be calculated as:
\[ \text{Average} = \frac{7.25 + 7.8 + X}{3} \]

3. **Set up the inequalities**:
Substitute the average into the inequalities:
\[ 7.1 < \frac{7.25 + 7.8 + X}{3} < 7.8 \]

4. **Multiply the entire inequality by 3** to eliminate the fraction:
\[ 3 \times 7.1 < 7.25 + 7.8 + X < 3 \times 7.8 \]
This simplifies to:
\[ 21.3 < 15.05 + X < 23.4 \]

5. **Isolate \( X \)** in both parts of the inequality:
- For the left side:
\[ 21.3 - 15.05 < X \implies 6.25 < X \]
- For the right side:
\[ X < 23.4 - 15.05 \implies X < 8.35 \]

6. **Combine the inequalities**:
Thus, we have:
\[ 6.25 < X < 8.35 \]

### Conclusion:
The range of the pH for the third reading that will keep the acidity level normal is:
\[ (6.25, 8.35) \]