Mass per unit length, \(\mu=0.001 \mathrm{kg} / \mathrm{m}\)
Frequency, \(v=\frac{1}{2 l} \sqrt{\frac{T}{\mu}}\)
\({v_{1}=\frac{1}{2 \times 0.516} \sqrt{\frac{20}{0.001}}}\)
\({v_{2}=\frac{1}{2 \times 0.491} \sqrt{\frac{20}{0.001}}}\)
\(\therefore \quad\) Number of beats \(=v_{1}-v_{2}=7\)