A string fastened at both ends resonates at $420 \mathrm{~Hz}$ and $490 \mathrm{~Hz}$ with no resonant frequencies in between. Find its fundamental resonant
frequency.
In general, $f_{n}=n f_{1}$. We are told that $f_{n}=420 \mathrm{~Hz}$ and $f_{n+1}=490 \mathrm{~Hz}$. Therefore,
$$
420 \mathrm{~Hz}=n f_{1} \text { and } 490 \mathrm{~Hz}=(n+1) f_{1}
$$
Subtract the first equation from the second to obtain $f_{1}=70.0 \mathrm{~Hz}$.