During a mass drill exercise, 6250 students of different schools are arranged in rows such that the number of students in each row is equal to the number of rows. In doing so, the instructor finds out that 9 children are left out. Find the number of children in each row of the square.

Answer

To solve the problem, we need to find the number of students in each row when 6250 students are arranged in such a way that the number of students in each row is equal to the number of rows, with 9 students left out.

### Step-by-step Solution:

1. **Understand the Problem**: We have 6250 students, and when arranged in rows, 9 students are left out. This means the number of students that can be arranged is \( 6250 - 9 = 6241 \).

2. **Set Up the Equation**: Let \( x \) be the number of students in each row. Since the number of rows is also \( x \), the total number of students that can be arranged in rows is \( x^2 \). Therefore, we can set up the equation:
\[ x^2 = 6241 \]

3. **Calculate the Square Root**: To find \( x \), we need to calculate the square root of 6241:
\[ x = \sqrt{6241} \]

4. **Finding the Square Root**: We can calculate the square root:
\[ x = 79 \]

5. **Conclusion**: Therefore, the number of students in each row is 79, and since the number of rows is also equal to the number of students in each row, there are 79 rows as well.

### Final Answer:
The number of children in each row is **79**.