variable,numer behaviour

  Hi all. I've tried the following:

(%i1) a:tan(%pi/23);
                                       %pi
(%o1)                              tan(---)
                                       23
(%i2) b:10+a;
                                     %pi
(%o2)                            tan(---) + 10
                                     23
(%i3) c:d/a;
                                      d
(%o3)                              --------
                                       %pi
                                   tan(---)
                                       23
(%i4) d:b;
                                     %pi
(%o4)                            tan(---) + 10
                                     23
(%i5) c,numer;
                                              %pi
(%o5)                  7.275540322183334 (tan(---) + 10)
                                              23

  Maybe i'm wrong, but when i use the ",numer" option i'm expecting a
numerical output (like //N in mathematica, to be clear). This is not
happening. Why? And, more importantly, how can i force a numerical
output?

thanks,
  Giovanni