/usr/lib/mono/4.0/System.Numerics.dll is in libmono-system-numerics4.0-cil 2.10.8.1-1ubuntu2.3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/mono/4.0/../gac/System.Numerics/4.0.0.0__b77a5c561934e089/System.Numerics.dll