# What is the period of a ball tied to a string of length 1.2 m making 4.5 revolutions every second?

I found $0.22$ seconds.
The period is the amount of time needed to complete a revolution. Your ball makes $4.5$ revolutions in $1$ second so it will need:
$\frac{1}{4.5} = 0.22$ seconds for only one revolution.