Question : A person from the top of a hill observes a vehicle moving towards him at a uniform speed. It takes 10 minutes for the angle of depression to change from $45^{\circ}$ to $60^{\circ}$. After this, the time required by the vehicle to reach the bottom of the hill is:
Option 1: 12 minutes 20 seconds
Option 2: 13 minutes
Option 3: 13 minutes 40 seconds
Option 4: 14 minutes 24 seconds
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Correct Answer: 13 minutes 40 seconds
Solution : Let, $CD$ = 10 units In $\Delta ABC$, $\tan 45^{\circ} = \frac{AB}{BC}$ $\Rightarrow BC = AB \quad................(1)$ In $\Delta ABD$, $\tan 60^{\circ} = \frac{AB}{BD}$ $\Rightarrow \sqrt{3} = \frac{AB}{BD}$ $\Rightarrow AB = \sqrt{3} BD$ $\Rightarrow BC = \sqrt{3} BD \quad [\text{using (1)}]$ $\Rightarrow BC = BD + CD$ $\Rightarrow \sqrt{3} BD - BD = CD$ $\Rightarrow BD(\sqrt{3} - 1) = 10$ $\Rightarrow BD = \frac{10}{\sqrt{3} - 1} \times \frac{\sqrt{3} + 1}{\sqrt{3} + 1}$ $\Rightarrow BD = \frac{10(\sqrt{3} + 1)}{2}$ $\Rightarrow BD = 5(1.732 + 1)$ $\Rightarrow BD = 5 \times 2.732$ $\Rightarrow BD = 13.66$ units $\therefore $ Time required to travel 10 units = 10 minutes $\Rightarrow$ Time required to travel 13.66 units = 13.66 minutes = 13 minutes 40 seconds approx. Hence, the correct answer is 13 minutes 40 seconds.
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