<blockquote>wget http://apt-cyg.googlecode.com/svn/trunk/apt-cyg

chmod +x apt-cyg

mv apt-cyg /usr/local/bin/

apt-cyg install bc</blockquote>
via <a href="http://superuser.com/questions/41093/installing-cygwin-packages-from-the-command-line">Installing Cygwin packages from the command-line - Super User</a>.

This google search was very useful!