
Each fork produces 4 beats per second with the previous means each fork has frequency 4Hz more than the previous.
Using relation, flast=ffirst+(N−1)x, here, N is the number of tuning fork in series and x is beat frequency between two successive forks.
2f0=f0+(20−1)4
⇒2f0=f0+76⇒f0=76Hz
Frequency of last tuning fork is 152Hz.