# Missing number in a 5x5 table

I'm trying to find a solution for the following puzzle. A friend sent it to me and I know that he didn't create it and that's about it for the original source; I tried to find it on the net but I couldn't make it.

Here it goes:

$$\begin{array}{|c|c|c|c|c|}\hline10&9&6&12&3\\\hline7&14&11&7&21\\\hline3&8&2&1&4\\\hline1&6&4&2&9\\\hline13&9&11&16&\color{red}?\\\hline\end{array}$$

I tried with sums, differences, products, ratio and a difference table but I couldn't find a solution.

• How do you know it's about patterns and calculations? Why not cipher for instance? – Arnaud Mortier Sep 15 '19 at 6:56
• I assumed it was because I exchange only puzzles of this kind with my friend. But you're right, I can't rule out your hypothesis. – D.K. Sep 15 '19 at 7:01