NCERT Figure it Out 3 - Q1 - Tales by Dots and Lines
Question: Mean grids: (i) Fill a 3 x 3 grid with 9 distinct numbers so every row, column, and diagonal averages 10. (ii) Can you change a few numbers and still keep average 10 in all directions?
Answer: Example grid shown; other distinct fillings with line sum 30 also work.
Full step-by-step NCERT solution with diagrams on Something by Zero.