Answer to Problem # 19:

Make a simple matrix table: 

                             NOW               THEN 

       JOE           24         Old as Bill is (Y)

       BILL          Y          ½ Joe's age now (12) 

              Joe's age minus Bill's age remains constant 
                 (Difference now = Difference then)

                24 - Y   =  Y - 12        or     2Y = 36 
                 
                 Y = 18 (Bill's age) (ANSWER) 

          'Then' is 6 years ago when Joe was 18 (as old as Bill is now) 


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