jmc

First let's convert $6.\overline{6}$ to a fraction. Let $p=6.\overline{6}$ and multiply both sides of this equation by 10 to obtain $10p=66.\overline{6}$. Subtracting the left-hand sides $10p$ and $p$ as well as the right-hand sides $66.\overline{6}$ and $6.\overline{6}$ of these two equations gives $9p=60$, which implies $p=20/3$. As we know, $0.60=6/10=3/5$. We multiply the two fractions to get $$\frac{\text{cancelto}420}{\cancel{3}}\cdot \frac{\cancel{3}}{\cancel{5}}=4.$$Voldemort had to pay \boxed{\4}$ for the ice cream.