According to the given question, 

the tuning forks will having frequencies in an arithmetic progression.

The nth fork's frequency is

${\mathrm{n}}^{\text{th}} \text{term}=\mathrm{a}+(\mathrm{n}-1)\mathrm{d}$

$f_n=f+(41-1)5=f+40$

From the question 

$f_n=2f=f+200$

$f=200Hz$