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

Question

$\begin {array} {1 1} (1)\;55\: cm & \quad (2)\;40\: cm \\ (3)\;60\: cm & \quad (4)\;70\: cm \end {array}$