Q:  What is the mass of one inch of this ruler?

        Mass of one inch  =  (length) * (linear density)

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

                          =   60 g

        

  Q:  What is the total mass of this ruler?

        Mass  =  (60 g) + (60 g) + (60 g) + ....

              =   6 * (60 g)  

              =   360 g