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

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

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

                          =   lambda grams

        

  Q:  What is the total mass of this ruler?

        Mass  =  (number of inches) * (mass of one inch)

              =           L         *    lambda grams

              =   (L * lambda)  grams