Two poles standing on horizontal ground are of heights 10 meters 40...

given,& $AB = 10, CD = 40$
$∠DBE = {30}^{o}$
in $\Delta BDE$,

$\dfrac{DE}{BE} = tan {30}^{o}$
$BE = \dfrac{DE}{\tan 3{0}^{o}} = 30\left(\dfrac{1}{3}\right) = 30\sqrt{3}$ meters