Tic-Tac-Toe Fifteen in a Row

The variation of tic-tac-toe I came up with a way to incorporate math into trying to win, it's fairly simple but became very intense and complex when people played.

The rules are as follows:
> The goal is to line up a set of 3 different numbers down, across, or diagonally that add up to 15
> One player uses only odd numbers while the other uses even (the player using odd numbers must go first since there are more odd numbers from 1-9 than even)
> Only numbers 1-9 can be used and the same number cannot be used twice


While trying out this variation we attempted a number of different ways to play the most effective of which was adding a time limit to each move, that put a lot more pressure on the player since there's less time to think about where to move and made them prone to mistakes that could cause them to lose. We experimented with other options, a different goal number, increased the range of number values you could use, and each player being able to use any number instead of just even or odd. Ultimately however the original rules still held up the best, the time limit addition was the only one that seemed to make the game more interesting. Overall I was very pleased with how this tic-tac-toe variation turned out and how it challenge players to think extremely carefully about every move.