official-package-repositories.list for linut mint 20.0
lwk@qwfys:~$ cat /etc/apt/sources.list.d/official-package-repositories.list
deb http://mirrors.tuna.tsinghua.edu.cn/linuxmint ulyana main upstream import backport
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu focal main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-updates main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-backports main restricted universe multiverse
deb http://mirrors.tuna.tsinghua.edu.cn/ubuntu focal-security main restricted universe multiverse
deb http://archive.canonical.com/ubuntu focal partner
lwk@qwfys:~$