Question

If 20 beats are produced within a single second, which of the following frequencies could possibly be held by two sound waves traveling through a medium along a common path at the same time
A. 38 Hz and 63 Hz
B. 18 Hz and 26 Hz
C. 47 Hz and 55 Hz
D. 22 Hz and 42 Hz

Answer 1

If 20 beats are produced within one second the frequencies that could possibly be held by two sound waves traveling through a medium along a common pants at the same time are b. 22  and 42 

Answer 2

The correct choice is

D. 22 Hz and 42 Hz.

In fact, the beat frequency is given by the difference between the frequencies of the two waves:

$f_B = |f_1 -f_2|$

In this problem, the beat frequency is $f_B=20 Hz$, therefore the only pair of frequencies that gives a difference equal to 20 Hz is

D. 22 Hz and 42 Hz.