The formula side of things probably isn't the challenge here so much as the math, but I think I can help.
If you know that the figure this month is 23% higher than last month's, it can be said that this month's figure is 123% of last month's figure. This means you can get 1% of last month's figure by dividing this month's by 123: 15,002.56 / 123 = 121.9720325203252
Now you can easily calculate last month's figure by simply calculating: 121.97 * 100 = 12197.20325203252
...or rounded down to 2 decimal places: 12197.20
Calculating this in Excel should be easy enough if you have an understanding of simple formulas. The formula will look something like this:
The above formula uses the ROUND function to round the result to 2 decimal places.
For more on rounding and the ROUND function, see the video and e-book lesson: Essential Skills Lesson 4-3: Format numbers using built-in number formats.
Aside from the ROUND function, this is a simple mathematical formula. For an introduction to creating formulas like this one, see the video and e-book lesson: Essential Skills Lesson 2-12: Create your own formulas.
I have attached an example spreadsheet showing an example of this formula in action.
Please feel free to reply if you need any more assistance with this.
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