If $0<a<20$, then $\frac{1}{a}>\frac{1}{20}$. Therefore, $\frac{1}{15}>\frac{1}{20}$ and $\frac{1}{10}>\frac{1}{20}$. Also, $\frac{1}{20}=0.05$ which is less than both 0.5 and 0.055. Lastly, $\frac{1}{20}>\frac{1}{25}$ since $0<20<25$. Therefore, $\frac{1}{25}$ is the only one of the choices that is less than $\frac{1}{20}$.