$2$ edge triggered T Flip Flops can be used to do it.
Given clock frequency $= 80 \ MHz$,
desired clock frequency $= \frac{1}{50 \times 10^{-9}} \ Hz = 20 \ MHz$,
so we have to divide given clock frequency by $4$ in order to achieve the desired clock frequency.
This can be done with the help of two edge triggered $T$ flip flops as follows:
I am using -ve edge triggered flip flops here.
$T$ input of both the flip flops is $1$.
Clock pulses from the given clock generator are passed to the first flip flop.
Output of first flip flop i.e. $Q1$ will change only on the -ve edges of given clock pulse, so it can be seen from the pulse plot that frequency of $Q1$ will be half of that of given clock pulse.
So $Q1$ will produce pulse of frequency $40 \ MHz$.
$Q1$ is being fed to the clock input of second flip flop.
And the output of second flip flop i.e. $Q2$ will change only on -ve edges of $Q1$,so the frequency of pulse produced by $Q2$ will be half of the pulse produces by $Q1$ and quarter of the pulse produced by the given clock generator.
Thus the frequency of the pulse produced by $Q2 = \frac{80}{4} = 20 \ MHz$.
Hence $Q2$ if used as a clock, will provide the clock cycle time of $50 \ ns$.
