20 tuning forks are arranged in series that each fork given 4 beat per second with the previous one. The frequency of the 20th fork is 3 times that of the first what is the frequency of the first tuning fork.

Explanation

Frequencies are arranged in increasing order frequency of the `P^th` fork is
` n_0=n_1+(p-1)n`
`3n_1=n_1+(20-1)xx4 `
`therefore [n=4 ,p=20 ,n_p=3n_1]`
`2n_1=19xx4 or n_1=`38hz