this must be a common problem, but repeated searching is not finding an answer

[Eliot Moss]

bc is available in Cygwin.

Note that Cygwin is not a single thing, but a library that provides a pretty
faithful mapping from POSIX library calls to Windows ones, PLUS a large number
of packages, individually installable (modulo their dependencies on one
another), of Posix compatible tools recompiled for Cygwin (often with patches
to make them compile properly under Cygwin).

bc is (logically enough) in the bc package.

Regards - Eliot Moss