Question 1169111
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At any given time, all the sun's rays are pointing at the same angle.<br>
So, assuming level ground, the ground, the sun's rays, the person, and the lamppost form similar triangles.<br>
In similar triangles, the ratios of corresponding sides are all the same.  A shadow 3 times as long (18 feet instead of 6 feet) means the object is 3 times as tall.<br>
Do the arithmetic....<br>