Ans!!

Your question is incomplete as you have not mention what is to be find out.

So, if your question is: 

A man on the top of a vertical tower observes a car moving at a uniform speed coming directly towards it . if it takes 12 min for the angle of depression to change from 30degree to 45degree , how soon after this will the car reach the observer tower

Then the solution is as follows:

 

 

 

Let AB be the vertical tower and P and Q be the position of the car at  the angle of depression 30° and 45°.

 

Let AB = h and 

PQ = x

Then 

∠ APB = 30° and ∠ AQB = 45°

 

Now

In Δ ABQ,

 

tan 45° = 

⇒BQ = h

 

In Δ APB

tan 30° = 

Hence, the time to reach the tower =16.39 minutes.

 

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