Answer: 4 feet
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Work Shown:
See below for the diagram.
(person height)/(person shadow) = (tree height)/(tree shadow)
x/3 = 6/4.5
x*4.5 = 3*6
4.5x = 18
x = 18/4.5 ...... dividing both sides by 4.5
x = 4
The boy is 4 feet tall.