GNU/Linux: How to Install libgfortran3 on Ubuntu 20+

If you encounter the error 'Unable to locate package libgfortran3', it means that the package is not available in your current package repository. This may be because you are using a newer version of Ubuntu that no longer supports libgfortran3.

In this case, you can download and install libgfortran3 manually from a trusted source, such as the official Ubuntu package archive. Here are the steps to do so:

1. Go to the Ubuntu packages website: https://packages.ubuntu.com/
2. Search for 'libgfortran3' in the search bar.
3. Select your Ubuntu version from the drop-down menu and click 'Search'.
4. Click on the link for the libgfortran3 package that matches your system architecture (e.g. amd64 for 64-bit systems).
5. Download the .deb file for the package.
6. Open a terminal and navigate to the directory where you downloaded the .deb file.
7. Install the package using the following command: sudo dpkg -i libgfortran3_*.deb (replace * with the actual version number of the package).

Once libgfortran3 is installed, you can proceed with installing the other dependencies and the desired software or package.