The fundamental frequency of a closed organ pipe of length

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The fundamental frequency of a closed organ pipe of length 20 cm is equal to the second overtone of an organ pipe open at both the ends. The length of organ pipe open at both the ends is

Fundamental frequency of closed organ pipe

V_c =	V
	4l_c

Fundamental frequency of open organ pipe

V₀ =	V
	2l₀

Second overtone frequency of open organ pipe

=	3V
	2l₀

From question,

=	V	-	3V
	4l_c		2l₀

⇒ l₀ = 6l_c = 6 × 20 = 120 cm