Step-by-step explanation:
speed = distance/time period
in our case we know speed = 216 km/hr
as it was the average velocity, we can assume the ball had the same velocity (meaning the same velocity ratio) during its trip from the time of getting hit and the time when hitting the ground. the distance of this trip was 20m
x = the time the ball needed to travel these 20 meters.
so, by keeping the same ratio, we know
216/1 km/hr = 20/x m/sec
as I can see that the trip of the ball was a matter of merely seconds (so, we will measure that time interval in seconds and not hours).
remember,
1 km = 1000 m
1 hour = 60 minutes
1 minute = 60 seconds
therefore,
1 hour = 60×60 = 3600 seconds
and so (we want to compare apples with apples, and not apples with oranges, so we want to compare m/sec with m/sec, and not km/hr with m/sec),
216/1 km/hr = 216000/3600 m/sec =
= 2160/36 m/sec = 60/1 m/sec
and now we can compare the ratios to calculate x :
60 = 20/x
60x = 20
x = 20/60 = 2/6 = 1/3 seconds = 0.333333... seconds
the elapsed time was 1/3 = 0.333333... seconds.