libmpfr-dev binary package in Ubuntu Focal i386

 This development package provides the header files and the symbolic
 links to allow compilation and linking of programs that use the libraries
 provided in the libmpfr4 package.
 .
 MPFR provides a library for multiple-precision floating-point computation
 with correct rounding. The computation is both efficient and has a
 well-defined semantics. It copies the good ideas from the
 ANSI/IEEE-754 standard for double-precision floating-point arithmetic
 (53-bit mantissa).

Source package