Time $= t\;s$

Distance $= xm$

$y=A \cos 240(t-x / 12)$

Distance $=0.5 m$

$y = A \cos 240\left(\frac{240 t-240 x }{12}\right)$

To find the wavelength,

$k =240 / 12$

$k =20$

$k$ is Wave number.

$\lambda=2 \pi / 20$

$\lambda$ is the wavelength.

$0.5 / \lambda=\theta / 2 \pi$

$0.5 /(2 \pi / 20)=\theta / 2 \pi$

$\theta=10^0$

The phase difference between two positions $0.5 m$ apart is $10^{\circ}$