A florist sold 15 arrangements in its first month of business. The number of arrangements sold doubled each month. What was the total number of arrangements the florist sold during the first 9 months?

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Answer 1 · 2018-04-16T07:55:54

7665 arrangements

We have a geometric series since a values is multiplied by a number each time (exponential). So we have #a_n=ar^(n-1)#

The first term is given as 15, so #a=15#.

We know it doubles each month, so #r=2#

Sum of a geometric series is given by:
#S_n=a_1((1-r^n)/(1-r))#

#S_9=15((1-2^9)/(1-2))=15(-511/-1)=15(511)=7665#