Answer
The third longest wavelength is $~~80~cm$
Work Step by Step
We can find the possible wavelengths of the standing waves:
$\lambda = \frac{2L}{n}$
$\lambda = \frac{2L}{1}, \frac{2L}{2}, \frac{2L}{3}, ...$
$\lambda = \frac{(2)(120~cm)}{1}, \frac{(2)(120~cm)}{2}, \frac{(2)(120~cm)}{3}, ...$
$\lambda = 240~cm, 120~cm, 80~cm, ...$
The third longest wavelength is $~~80~cm$
