The third harmonic of a guitar string has a frequency of 170 hz. what is the length of the guitar string if the speed of the waves on the string is 125 m/s?
1.103 m
2.206 m
0.368 m
2.040 m

Refer to the figure shown below.

At the 3rd harmonic, there will be 3/2 wavelengths over the length of the guitar string. That is,
L = (3/2)λ
where
L = length of guitar string
λ = wavelength

Note that v = fλ, where
v =  wave velocity,
f =  wave frequency,.

Therefore
L = (3/2)λ
   = (3/2)*(v/f)
   = (3/2)*(125 m/s)/(170 1/s)
   = 1.1029 m

1.103 m