79. Children are doing exercises on the playground, Xiao Jun stands in the 3rd column from the left, and the 3rd column from the right; counting from the front, he is in the 3rd row, and counting from the back, he is in the 3rd row. If the number of people in each row is the same, and the number of people in each column is the same, then there are $\qquad$ children doing exercises.
answer: 25