Q:  What is the mass of this little piece?

        mass of piece     =  (length) * (linear density)

                          =  (1 inch) * (60 g/inch)

                          =   60 g


  Q:  What is the contribution of this little piece
          to the equation for center of mass?

        (mass) * (position)   =  (60 g) * (1.5 inch)

                              =     90 g*inch
        


  Q:  What is the location of the center
        of mass of the ruler in the horizontal direction?


              (60 g)*(0.5 inch) + (60 g)*(1.5 inch) + (60 g)*(2.5 inch) + ...
   X com  =   ----------------------------------------------------------
                             (60 g) + (60 g) + (60 g) + ....


                   1080 g*inch
              =   -------------   =   3 inch
                     360 g