A source of sound producing wavelength of 50 cm is moving away from stationary observer with $\large\frac{1}{5}th$ speed of sound. Then, what is the wavelength of sound heard by observer ?

Question

$\begin {array} {1 1} (1)\;70\: cm & \quad (2)\;50\: cm \\ (3)\;40\: cm & \quad (4)\;60\: cm \end {array}$