symbol evaluation order



Hello,
I am a maxima beginner, and I don't quite understand how expressions are
evaluated. In the example session below (calculating the volumen V of a
sphere with radius r, diameter d), the final evaluation ''V results in an
expression containing d, although d is defined.

What should I have typed, so that the value of d is substituted in the
evaluation? Of course I could write ''V,d=d; but I would like to have all
known symbols substituded recursively (for more complicated examples).

Thanks
Brian

(%i1) V:4/3*%pi*r^3;
                                          3
                                   4 %pi r
(%o1)                              --------
                                      3
(%i2) r:d/2;
                                       d
(%o2)                                  -
                                       2
(%i3) d:20;
(%o3)                                 20
(%i4) ''V;
                                         3
                                    %pi d
(%o4)                               ------
                                      6
(%i5)  
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