A secret can be told to only 3 persons in 3 minutes. Each person in turn tells 3 other persons in the next 3 minutes and the processes continues accordingly. In 30 minutes how many persons can be told this secret in this way ?

Given a secret is told to 3 persons in 3 minutes.
And if the process continues with each person saying 3 other persons,
In 30 minutes => the rocess for 10 times because each person time is 3 minutes.
Required number of persons =  $3 + 32 + 33 + .... + 39 + 310$

Now it forms a geometric progression,

Sum to n terms is $S = \frac{arn - 1}{r - 1}$

Here a = 3, r = 3

 
=> 59048 x 3 /2

= 88572.