First, we are going to draw a picture for the problem
We can formulate the following equation, given that we can use triangle similarity theorem.
[tex]\frac{\text{Andy height}}{x\text{ (post height)}}=\frac{\text{shadow of Andy}}{\text{Shadow of the post}}[/tex]Replacing the values we have
[tex]\begin{gathered} \frac{5}{x}\text{ = }\frac{9}{35} \\ 5\cdot\frac{35}{9}=x \\ 19.4\text{ = x} \end{gathered}[/tex]Isolating x, we get x is equal to 19.4 ft, which is the height of the post.
