A set of 20 tuning forks is arranged in a series of increasing...

Each fork produces $4$ beats per second with the previous means each fork has frequency $4Hz$ more than the previous.

Using relation, ${f}_{last} = {f}_{first} + \left(N - 1\right)x$, here, $N$ is the number of tuning fork in series and $x$ is beat frequency between two successive forks.

$2{f}_{0} = {f}_{0} + \left(20 - 1\right)4$

$⇒ 2{f}_{0} = {f}_{0} + 76 ⇒ {f}_{0} = 76Hz$

Frequency of last tuning fork is $152Hz$.