Answer
$30$ m/s
Work Step by Step
Horizontal distance during interval $T$ is equal to ( Horizontal speed ) $\cdot$ duration of travel.
Since, the horizontal speed of ball is constant$=v_0 \cos 45^\circ=\dfrac{v_0}{\sqrt 2}$
Now,
$90=\dfrac{v_0}{5 \sqrt 2} \times \dfrac{v_0}{\sqrt 2}$
$90=\dfrac{v_0^2}{10}$
This implies
$v_0=30$ m/s
