a, 2 + ( -25 ) + 41 + ( -2 ) + 25 + ( -41 )
= 2 - 25 + 41 - 2 + 25 - 41
= ( 2 - 2 ) + ( 25 - 25 ) + ( 41 - 41 )
= 0.
b, 10 + ( -17 ) + 5 + ( -7 ) + 17 + ( -15 )
= 10 - 17 + 5 - 7 + 17 - 15
= ( 17 - 17 ) + ( 10 + 5 - 15 ) - 7
= 0 + 0 - 7
= -7.
c, ( -22 ) + ( -14 ) + 17 + ( -24 ) + 13 + 30
= -22 - 14 + 17 - 24 + 13 + 30
= ( 17 + 13 + 30 ) - ( 22 + 14 + 24 )
= 60 - 60
= 0.