# How to calculate sum of series (Excel FV function?) **URL:** \ **Category:** Factual Questions\ **Created:** [September 21, 2019, 9:02pm UTC](https://boards.straightdope.com/t/how-to-calculate-sum-of-series-excel-fv-function/840625 "2019-09-21T21:02:36Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![K364](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k364/32/5_2.png) [@K364](https://boards.straightdope.com/u/K364)\ **Post date:** [September 21, 2019, 9:02pm UTC](https://boards.straightdope.com/t/how-to-calculate-sum-of-series-excel-fv-function/840625/1 "2019-09-21T21:02:36Z")
In the game Cookie Clicker (don’t go there unless you want to be sucked into the universe’s biggest time waster) you buy buildings. Each building you buy will be 15% more expensive than the previous. I want to know if the current cost is X, how much will N buildings cost? The game has a feature that will tell you how much 10 buildings will cost. In Excel I duplicated this result with X + 1.15X + 1.15_1.15X + 1.15_1.15\*1.15X … etc The game will tell you the cost of 1, 10, or 100. I want to know 50 or 17 or whatever. I tried the Excel FV function and couldn’t get it to work. Any help would be greatly appreciated! ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [September 21, 2019, 9:12pm UTC](https://boards.straightdope.com/t/how-to-calculate-sum-of-series-excel-fv-function/840625/2 "2019-09-21T21:12:28Z")
It’s a geometric series, looks like : The sum is = X (1.15^n -1)/(1.15 -1) where n is the number of terms. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 21, 2019, 9:22pm UTC](https://boards.straightdope.com/t/how-to-calculate-sum-of-series-excel-fv-function/840625/3 "2019-09-21T21:22:59Z")
Or, for a quick rule of thumb, 50 of a building will cost approximately 1000 times as much as buying one, and buying 5 more will cost about as much as all of the buildings of that type you’ve already bought combined. ---
**Author:** ![K364](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k364/32/5_2.png) [@K364](https://boards.straightdope.com/u/K364)\ **Post date:** [September 21, 2019, 10:30pm UTC](https://boards.straightdope.com/t/how-to-calculate-sum-of-series-excel-fv-function/840625/4 "2019-09-21T22:30:05Z")
got it, thanks! ---
**Author:** ![SamuelA](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@SamuelA](https://boards.straightdope.com/u/SamuelA)\ **Post date:** [September 22, 2019, 1:34pm UTC](https://boards.straightdope.com/t/how-to-calculate-sum-of-series-excel-fv-function/840625/5 "2019-09-22T13:34:24Z")
> [@K364](#): > > In the game Cookie Clicker (don’t go there unless you want to be sucked into the universe’s biggest time waster) you buy buildings. Each building you buy will be 15% more expensive than the previous. I want to know if the current cost is X, how much will N buildings cost? > > The game has a feature that will tell you how much 10 buildings will cost. In Excel I duplicated this result with X + 1.15X + 1.15_1.15X + 1.15_1.15\*1.15X … etc > > The game will tell you the cost of 1, 10, or 100. I want to know 50 or 17 or whatever. > > I tried the Excel FV function and couldn’t get it to work. Any help would be greatly appreciated! Hey if you want to try a version of this game that is (1) got a fun and moderately thought provoking story (2) solvable in a single session Try paperclip maximizer: [http://www.decisionproblem.com/paperclips/](http://www.decisionproblem.com/paperclips/)