Two balls A and B are projected from the same position with...

\overset{\to}{{v}_{1}} = 5\overset{^}{i} - gt\overset{^}{j} ⇒ {u}_{1} = 5

\overset{\to}{{v}_{2}} = 20\overset{^}{i} - gt\overset{^}{j} ⇒ {u}_{2} = 20

As $\overset{\to}{{v}_{1}}$ and $\overset{\to}{{v}_{2}}$ are perpendicular, 

$\overset{\to}{v}1 \cdot \overset{\to}{v}2 = 0$
$t = \dfrac{\sqrt{{u}_{1}{u}_{2}}}{g}$
${\overset{\to}{s}}_{\text{rel}} = {\overset{\to}{u}}_{\text{rel}} \times t$
$= ( {u}_{1} + {u}_{2} ) t$
$= \left({u}_{1} + {u}_{2}\right)\dfrac{\sqrt{{u}_{1}{u}_{2}}}{g}$
$= 25 \times \dfrac{10}{10} = 25m$