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## Homework Statement

A boy is walking away from a wall towards an observer at a speed of 1 m/s and blows a whistle whose frequency is 680 Hz. THe number of beats heard by the observer is ____.

(Velocity of sound in air=340 m/s)

a) zero

b)2

c)8

d)4

## Homework Equations

## The Attempt at a Solution

For this question, I used f(observer)=f(source)v/(v-v(source)). In this case, when i plugged in the number i got 682 Hz...but the only thing i can think of is to subtract 682 from 680 Hz which gives 2...i don't know why if it's right though...