Let's assume
the number of children in each row of the square is x
Since, the number of students in each row is equal to the number of rows
so, the number of children in each column of the square is x
6250 students of different schools
But 9 children are left out
so, total number of student in rows and columns =6250-9=6241
total number of student in rows and columns=(the number of children in each row of the square)*(the number of children in each column of the square)
now, we can plug values


now, we can solve for x


So,
the number of children in each row of the square is 79...........Answer