jmc

If the interest rate is $r$, it follows that $$20000(1+r{)}^2+20000(1+r)+20000\ge 66200.$$ If we set $x=1+r$ and divide through the inequality by $200$, it follows that $$100x^2+100x-231\ge 0.$$ Since $231=11\cdot 21$, we can factor the quadratic as $(10x-11)(10x+21)\ge 0$, so it follows that $x\ge \frac{11}{10}$ or $x\le \frac{-21}{10}$. Since we are looking for an interest rate percentage, it follows that $x\ge \frac{11}{10}=1.1$, and $r=x-1=\boxed{10}\%$.