I am trying to solve this question-

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> The driver of an engine produced a whistle sound from a distance $800m$ away a hill to which the engine was approaching.The driver heard the echo after $4.5s$.Find the speed of the car is speed of sound through air is $340 m/s$.
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My attempt-

[![enter image description here](https://i.stack.imgur.com/6Yttp.jpg)](https://i.stack.imgur.com/6Yttp.jpg)

$V=\frac {2d}{t}$ (2d since echo sound has to travel twice from A to B and then from B to A again.)

Putting $v=340m/s,d=800m$ we get,$t=\frac{80}{17}s$.

So,time taken to move $2AB$ (AB+AB)=$\frac{80}{17}s$.

Let,driver has moved to point O when sound reaches him at O.

So,time taken by sound to reach O (A to B and then from B to O)=4.5s.

So,2AB time-(AB+OB) time=$(\frac{80}{17})-4.5=\frac{7}{34}s$

So,AB-OB time=$\frac{7}{34}s.$

So,$\frac{40}{17}-OB=\frac{7}{34}s.$

Solving we get,time taken by sound to reach $OB=\frac{73}{34}s$. 

So,distance $OB=speed\times time=340\times \frac{73}{34} m.$

So,we can find AO and OB.

So,time taken by sound to reach from A to O by car=time taken by sound to reach from B to O.

applying time=distance/speed,and making the time taken by sound=that of car we get,

$\frac{AO}{Speed\_{car}}=\frac{OB}{Speed\_{sound}}.$

Solving we get,$Speed\_car=\frac{2380}{34} m/s=32 m/s$ approx.

But answer given is $15.5m/s$.Where am I going wrong?

Thanks for any help.