#### Answer

$x=5$ ft

#### Work Step by Step

Similar triangles have corresponding sides that are proportional.
The corresponding sides in the similar triangles are:
the 8-ft and the 4-ft sides
the 10-ft side and the one marked with an $x$.
Use proportions to obtain:
$\dfrac{8}{4} = \dfrac{10}{x}$
Cross-multiply to obtain:
$8x=4(10)
\\8x=40
\\x = \frac{40}{8}
\\x=5$