Q.

25 tuning forks are arranged in series in the order of decreasing frequency. Any two successive forks produce 3 beats/sec. If the frequency of the first tuning fork is the octave of the last fork, then the frequency of the 21st fork is

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a

72 Hz

b

288 Hz

c

84 Hz

d

87 Hz 

answer is C.

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Detailed Solution

According to the question frequencies of first and last tuning forks are 2n and n respectively.
Hence frequency in given arrangement are as follows
 

Question Image

Þ 2n – 24 ´ 3 = n Þ n = 72 Hz 
So, frequency of 21st tuning fork 
n21=(2×7220×3)=84 Hz

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