Assume that you have just heard of a scandal and you are the first one to know. You pass it on to four person in a matter of 30 minutes. Each of these four in turn passes it to four other persons in the next 30 minutes and so on.
How long it will take for everybody in the World to get to know the scandal?
Assume that nobody hears it more than once and the population of the World is approximately 5.6 billions.