Copy/ Paste is one of Excel’s best features. But be aware of what can go wrong, especially when it comes to absolute and relative referencing ($ signs in formula).
To follow along go to Copy&Paste HEH
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As shown in the video, and you will see with the rest of the lessons, making the mistake is easy.
Avoiding them involves checking that what you have done in Excel, is what you meant to do.
Use of the Auditing Toolbar (blue lines between cells) is critical. You can learn more about these Formula Auditing Blue lines in the free Excel Fundamentals Course.
The other knowledge that is critical is understanding the $ signs (absolute and relative referencing). This is also covered in the free Excel Fundamentals Course.