Answer
$0.205$ s
Work Step by Step
First,
we need to convert $161\dfrac {km}{h}$ to $\dfrac {m}{s}$. İn order to do this; we need the following data:
1hr=3600s;1km=1000m
Therefore,
$v_{0}=161\dfrac {km}{h}=161\dfrac {1000m}{3600s}\approx 44.72\dfrac {m}{s}$
This is the initial horizontal velocity of the ball and its horizontal velocity doesn't change since there is no horizontal force acting on the ball. So the time needed to travel half of this given distance will be: $t_{1}=\dfrac {L}{2v_{0}}=\dfrac {18.3m}{2\times 44.72\dfrac {m}{s}}\approx 0.205s$
