SOLUTION: A school has a telephone system for letting parents know about emergency school closing. The system is a pyramid with five layers. Three parents are on top layer of the pyramid. Ea

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Question 333459: A school has a telephone system for letting parents know about emergency school closing. The system is a pyramid with five layers. Three parents are on top layer of the pyramid. Each parent in each layer calls three different parents. How many parents are in the chain?
Answer by jrfrunner(365) About Me  (Show Source):
You can put this solution on YOUR website!
at layer k there are 3^k parents, where k=1 to 5
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total = sum+%283%5Ek%29 k=1 to 5
total =%283%5E%285%2B1%29-3%29%2F%283-1%29=%283%5E6-3%29%2F2=363
===
note:
In general
total = sum+%28r%5Ek%29 k=1 to n
total =%28r%5E%28n%2B1%29-r%29%2F%28r-1%29