In: Math
Log onto website where you can observe your service bill for the last 12 months (electric bill, cell phone bill, water bill, etc.). If you do NOT feel comfortable sharing this data, you can make up values.
Analysis
Solved below in Excel as asked. If you want to replicate the same with formulas the 2nd screenshot is of formulas of the 1st img
From the 3 selected bills the 51 and 494 values are above 2 std deviations and hence they are unusual
Below img has all the z values of all the bills and highlighted in Red are the unusual values
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