Ambivalent Fans

We can write our two fractions as $\frac{5}{14} = \frac{25}{70}$ and $\frac{9}{15} = \frac{42}{70}$ . So in total $\frac{67}{70}$ of the people were cheering for one team or the other. That means that $\frac{3}{70}$ of the people were not cheering for any team. Since $\left \lfloor \frac{3000}{70} \right \rfloor = 42$ , so at most there were $42 \times 70 = 2940$ people in the stands, with $\frac{3}{70} \times 2940 = 126$ of them not cheering for either team.