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4- 2- 1- : 800 36 26425 (48) 599 5 +- -+- - - 25 4 2 . 25 5 1250 25 5 . 1250 1250 25 5= 50 5= 10 1250 (25 5)= 1250 125= 10 12502551250(255)=a b ca ( b c )= - . .2000 125 8390 13 3 .2000 125 8= 2000 (125 8)= 2000 1000= 2 .390 13 3420 35 = 420 ( 7 5 ) = 420 7 5 = 60 5 = 12 a ( b c ) = a b ca b c = a ( b c )a ( b c )= a b c420 3 7420 (3 7) 3 420 .350 14 = 350 ( 7 2 ) = 350 7 2 350 . 14