A tuning fork of unknown frequency makes 5

Question:

A tuning fork of unknown frequency makes 5 beats per second with another tuning fork which can cause a closed organ pipe of length $40 \mathrm{~cm}$ to vibrate in its fundamental mode. The beat frequency decreases when the first tuning fork is slightly loaded with wax. Find its original frequency. The speed of sound in air is $320 \mathrm{~m} \mathrm{~s}^{-1}$.

Solution: