At what speed should a source of sound move away from a stationary observer so that observer finds the apparent frequency equal to half of the original frequency?

Explanation

When the source is moving away from the stationary listener, the expression for the apparent frequency is
`n.=((v)/(v+v_(s)))n`
`(n)/(2)=((v)/(v+v_(s)))n`
`v_(s)=v`