# Geometric series question

• Apr 8th 2010, 10:16 AM
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 :(
• Apr 8th 2010, 10:46 AM
Quote:

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 $n$ rounds, the number of people contacted (including the original $3$) is:
$3 + 3\times3+3\times3^2+3\times3^3...+3\times3^{n-1}$
which is a Geometric Series with:
first term, $a = 3$; common ratio $r = 3$.
The sum to $n$ terms, $S_n$ is given by:
$S_n = \frac{a(r^n-1)}{r-1}$
$=\frac{3(3^n-1)}{2}$
We want $S_n \ge 9840$. So:
$\frac{3(3^n-1)}{2}\ge9840$

$\Rightarrow 3^n-1\ge6560$

$\Rightarrow n = 8$