A 1.5 m tall boy is walking away from the base of a lamp post which is 12 m high, at the speed of 2.5 m/sec. Find the length of his shadow after 3 seconds.

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Step 1: Distance covered in 3 sec $= 7.5$ m. Let shadow length $= x$ m. By similar triangles: $\frac{1.5}{12} = \frac{x}{7.5 + x}$
Step 2: $x = \frac{15}{14}$ or $1.07$ m approx. Length of shadow $= 1.07$ m.

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