Two vertical poles AL and BM of heights 25m and 100m respectively...

$\dfrac{25}{x + y} = \dfrac{z}{y} \left\{∵ ∆ LAB ∆ PNB\right\}$
⇒ y = \dfrac{z\left(x + y\right)}{25} \ldots \left( i\right)$<br />Also, \dfrac{100}{x + y} = \dfrac{z}{x}\left\{∵ ∆ MBA ∆ PNA\right\}$
$⇒ x = \dfrac{z\left(x + y\right)}{100} \ldots \left(ii\right)$
Adding equation \left(i\right) & \left(ii\right)$, we get</p><p>$x + y = \left(x + y\right)\left(\dfrac{z}{25} + \dfrac{z}{100}\right)$⇒ z = 20$