/usr/lib/x86_64-linux-gnu/libmpfr.so is in libmpfr-dev 3.1.2-2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmpfr.so.4.1.2
/usr/lib/x86_64-linux-gnu/libmpfr.so.4.1.2