Thread: Geometric series question

A large school board established a phone tree to contact all of its employees in case of emergencies. Each of the 3 superintendents calls 3 employees who each in turn calls 3 other employees and so on. How many rounds of phone calls are need to notify all 9840 employees?

The back of the book says the answer is 8 rounds but I have no idea how to get that Please help

Hello needhelpplease1

Originally Posted by needhelpplease1

A large school board established a phone tree to contact all of its employees in case of emergencies. Each of the 3 superintendents calls 3 employees who each in turn calls 3 other employees and so on. How many rounds of phone calls are need to notify all 9840 employees?

The back of the book says the answer is 8 rounds but I have no idea how to get that Please help

After rounds, the number of people contacted (including the original ) is:

which is a Geometric Series with:

first term, ; common ratio .

The sum to terms, is given by:

We want . So:

Grandad