As it gives 4 beats per second when sounded with a source of frequency \(250 \mathrm{Hz}\)
\(\therefore \quad v=250 \pm 4=246 \mathrm{Hz}\) or \(254 . \mathrm{Hz}\)
Second harmonic of this unknown source\(=492 \mathrm{Hz}\) or \(508 \mathrm{Hz}\) which gives \(5\) beats per second, when sounded with a source of frequency \(513 \mathrm{Hz}\) Therefore unknown frequency, \(v=254 \mathrm{Hz}\).