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## Homework Statement

A girl 5 feet tall is running at the rate of 12 feet/second and

passes under a street light 20 feet above the ground. Find how

rapidly the length of her shadow is increasing when she is 20

feet past the base of the street light.

## The Attempt at a Solution

I let the distance of girl from light be S = 12t

I let the distance of shadow from light be X,

Due to similar triangle,

[itex]\frac{x}{x-12t} = \frac{20}{5}[/itex]

[itex]-15x=-240t[/itex]

[itex]x=16t[/itex]

[itex]\frac{dx}{dt}=16[/itex]

But the ans is 4ft/sec any clues as to where I went wrong?