A fish rising up vertically toward the surface of water with speed `3ms^(-1)` observes a bird diving down vertically towards it with speed `9ms^(-1)` . The actual velocity of bird is

Explanation

Velocity of bird w.r.t. to fish `v_(B//F)=9m//sec`
Velocity of fish w.r.t. to ground `v_(F//g)=3m//sec`
Let at some instant, situation is as shown in above diagram.
The distance between bird and fish, as seen by fish
`FB^'=r=x+muy`
Differentiating w.r.t.
`(dr)/(dt)=(dx)/(dt)+mu(dy)/(dt)`
`v_(B//F)=v_(F//g)+muv_(B//g)`
`9=3+4/3v_(B//g)`
`v_(B//g)=4.5m//sec`