Answer
$30$ m/s
Work Step by Step
As we know, recall the definition for horizontal distance which says that Horizontal distance during interval $T$ is equal to ( Horizontal speed ) $\cdot$ duration of travel.
As we are given that the horizontal speed of a ball is constant. This means that $=v_0 \cos 45^\circ=\dfrac{v_0}{\sqrt 2}$
Need to solve the problem.
$90=\dfrac{v_0}{5 \sqrt 2} \times \dfrac{v_0}{\sqrt 2}$
or, $90=(\dfrac{v_0}{5 \sqrt 2}) \times (\dfrac{v_0}{\sqrt 2})$
or, $90=(\dfrac{v_0^2}{10})$
Hence, $v_0=30$ m/s
