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Q.
The number of possible natural oscillations of air column in a pipe closed at one end of length $85 \,cm$ whose frequencies lie below $1250 \,Hz$ are (Velocity of sound $=340\, m s ^{-1}$ )
The fundamental frequency
$n=\frac{v}{4 l}=\frac{340 \times 100}{4 \times 85}=100 \,Hz$
and as only odd harmonics are heard in a pipe closed at one end, the possible frequencies are $100,300 , 500,700,900,1100,1300$, etc.
$\therefore$ The frequencies below $1250$ are
$100,300,500,700,900$ and $1100$ .
Thus the number of possible natural oscillations is $6$ .