You have an important message that needs to be delivered to seven other people. everyone who knows the message, can also spread it via phone, but only to one person at a time. conveying the message takes exactly one minute (dialing included). how long does it take until everyone is clued

In this case, the amount of people that know would be multiplied by two in every minute

1 person  - - - - - > 1 person      = 2 people know after the first minute

1 person > 1 person         
                                                   = 4 people know after the second minute
1 person > 1 person

After 3 minutes ,  all the people will know.

so the answer is 3 minutes

The correct answer is $90