A source of unknown frequency gives 4 beats/s when sounded with a source of known frequency 250 Hz. The second  harmonic of the source of unknown frequency gives five beats per second, when sounded with a source of frequency 513 Hz. The unknown frequency is

Let υ be frequency of the unknown source. As it gives 4 beats per second when sounded with a source of frequency 250 Hz,   ∴   υ=250±4=246Hz  or  254 Hz         Second harmonic of this unknown source = 492 Hz or 508 Hz  which gives 5 beats per second, when sounded with a source of frequency 513 Hz.Therefore unknown frequency, υ = 254 Hz.