Question

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Answer 1

Height of lamp post = 3.3 m = 330 cm. 

The distance covered by the boy in 4 s = 0.8 x 4 = 3.2 m = 320 cm. 

After 4 s, if we join the upper points of the lamp post and the boy's shadow, the larger triangle formed by the height of lamp post and the total distance obtained by adding the distance covered by the boy and the length of his shadow, will be similar to the smaller triangle formed by the boy's height and the length of his shadow. 

∴ Height of lamp post / Boy's height = Total distance of that covered by the boy and length of the shadow / Length of the shadow 

= $\frac{330}{110} = \frac{320 + x}{x} \\ \\ = \frac{320 + x}{x} = 3 \\ \\ 320 + x = 3x \\ 320 = 3x - x = 2x \\ x = \frac{320}{2} = 160$ 

∴ The length of the boy's shadow after 4 s is 160 cm. 

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