A rumour was spread by 4 man at 9:00am. In a period of 15 minutes,each of the 4 man told the rumour to another 3. For the following 15 minutes,each of the man told the rumour to another 3 who have yet to hear it. At 11:00am, how many man had heard it?
1 Answer
Explanation:
Note that there are
At time
the number of men with the rumour would be
At time
the number of men with the rumour would be
At time
the number of men with the rumor would be
In general at time
the number of men with the rumour would be
This is the sum of a standard geometric series with
We know that
Using the above values for
and with
we have