With what velocity an observer should move relative to a stationary source so that he hears a sound of double the frequency of source?

$\mathrm{By} \mathrm{using} {\mathrm{n}}^{\text{'}} = \mathrm{n}\left(\frac{\mathrm{v}}{\mathrm{v}-{\mathrm{v}}_{\mathrm{s}}}\right)$

$2\mathrm{n} = \mathrm{n}\left(\frac{\mathrm{v}-{\mathrm{v}}_0}{\mathrm{v}-0}\right) \Rightarrow {\mathrm{v}}_{\mathrm{o}} = -\mathrm{v} = -(\mathrm{Speed} \mathrm{of} \mathrm{sound})$

Negative sign indicates that observer is moving opposite to the direction of velocity of sound, as shown