Answer
Depth of the lake is $30m$
Work Step by Step
let $v$ be the velocity of ball with which it hits the water. So we have
$ v^2 - u^2 =2as_1$
$u=0$ ,$s_1 = 5.0m$ and $ a= g = 9.8ms^{-2}$
thus
$ v^2 - 0 = (2* 10 * 5 )m^2s^{-2} = 100m^2s^{-2}$
$v= 10ms^{-1}$
let $s_2$ be the depth of lake. therefore
$s_2 = v*t$
$v= 10ms^{-1}$ and $t=3.0s$, Thus
$s_2 = (10 * 3)m =30m$