\[\boxed{\mathbf{П.\ 20}}\]
\[n \cdot m = 105\]
\[n = \frac{105}{m}\]
\[\textbf{а)}\ m = 3:\]
\[n = \frac{105}{3} = 35\ см.\]
\[\textbf{б)}\ m = 15:\]
\[n = \frac{105}{15} = 7\ см.\]
\[\textbf{в)}\ m = 21:\]
\[n = \frac{105}{21} = 5\ см.\]
\[Да,\ зависимость\ обратно\ \]
\[пропорциональная.\]