jmc

The 8th grade has $\frac{650}{520+650}=\frac{650}{1170}=\frac{65}{117}$ of the total students. To simplify this fraction further, we notice that $65=5\cdot 13$. Since $117$ is not divisible by $5$, we test if it's divisible by $13$ and find that $117=9\cdot 13$. Thus to have fair representation, the 8th grade should have $\frac{65}{117}\times 18=\frac{5}{9}\times 18=\boxed{10}$ of the $18$ representatives.