Let the frequency of first fork =n then the frequency of last fork =3n Since two successive forks given four beats. Hence, we have Frequency of first fork =n Frequency of 2 nd fork =n+4 Frequency of 3rd fork =n+2×4 Frequency of 4 th fork =n+3×4 So, the frequency of N th fork =n+(N−1)4 But N th fork is the last fork ∴3n=n+(N−1)4 ⇒2n=(N−1)4 Here, N=56 ⇒2n=(56−1)×4=55×4=220 ⇒n=