a) ( 20 + 21 +22 + 23 + 24 +25 ) x ( 16 - 2 x 8 )
= ( 20 + 21 +22 + 23 + 24 +25 ) x ( 16 - 16 )
= ( 20 + 21 +22 + 23 + 24 +25 ) x 0
= 0
b) 1235 x 6789 x ( 630 - 315 x 2 ) : 1996
= 1235 x 6789 x ( 630 - 630 ) : 1996
= 1235 x 6789 x 0 : 1996 = 0
c) ( m : 1 - m x 1 ) : m x ( 2008 + m +2008 )
= ( m - m ) : m x ( 2008 + m +2008 )
= 0 : m x ( 2008 + m +2008 ) = 0