AIPMT 2010 Physics and Chemistry Paper

Let the frequencies of tuning fork and piano string be v1 and v2 respectively.
∴v2=v1±4=512‌Hz±4
=516‌Hz or 508‌Hz=516‌Hz or 508‌HzIncrease in the tension of a piano string increases its frequency.
If v2=516‌Hz, further increase in v2, resulted in an increase in the beat frequency. But this is not given in the question.
If v2=508‌Hz, further increase in v2 resulted in decrease in the beat frequency. This is given in the question. when the beat frequency decreases to 2 beats per second.
Therefore, the frequency of the piano string before increasing the tension was 508‌Hz.