Forty one tuning forks are arranged in increasing order of frequencies such that every fork gives 5 beats with the next. The last fork has frequency that is double the frequency of first fork. The frequency of the fork is

Frequency of first tuning fork = n (say) Frequency of second tuning fork $=(\mathrm{n}+5)=\mathrm{n}+1\times 5$ Frequency of third tuning fork 
$=\mathrm{n}+10=\mathrm{n}+2\times 5$  Frequency of fourth tuning fork $=\mathrm{n}+15=\mathrm{n}+3\times 5$ $\text{ Frequency of }{41}^{\text{th }}\text{ tuning fork }=\mathrm{n}+40\times 5$
$\text{ So, }\mathrm{n}+40\times 5=2\mathrm{n}\text{ or }\mathrm{n}=200\mathrm{Hz}$