The correct option (B) 138
Explanation:
Total no of forks = 24
each fork produces 6 beats per second with preceding fork.
freq of last fork = 2 × freq of first fork
Let frequency of 1st fork be f1
∴ freq of 2nd fork = f1 + 6
= f1 + 6(2 – 1)
freq of 3rd fork = f1 + 6 + 6
= f1 + 6(3 – 1)
similarly
freq of 24th fork = f1 + 6(24 – 1)
= f1 + 138
also freq of 24th fork = 2 × freq of 1st fork
∴ f1 + 138 = 2 f1
∴ f1 = 138 Hz