figure this out

a local farmer passed away recently and left 17 mules to his three sons. his will said the oldest son would get half. the second oldest son gets 1/3 and the youngest son gets 1/9 th. . you obviously cant divide 17 mules in half so they began to fight. one of their unkles saw them fighting and rode a mule up and put it with the other 17 making 18 total. then he said oldest son take your 9 ( half ) and leave . next son take your 6 ( 1/3 ) and leave..youngest son take your 2 ( 1/9 th ) and go. 9 + 6+ 2 = 17 . unkle got on his mule and rode home