first find the frequency heard...
speed of sound at 20 deg C = 343 m/s
f = v / λ = 343 /0.60 = 571.7
Hz
Then... find the wavelength of the wave on the string:
λ = 2L = 2 * 0.40 = 0.80 meters
Find the speed of the waves on the
string: speed = λ f = 0.80 * 571.7
= 457.3 m/s
Then use: speed2 = Tension / μ or
Tension = speed2 *μ =
457.32 * 0.001 = 209.2 Newtons