5 - 10 + 15 - 20 + 25 - 30 + 35 - 40 + 45 - 50 + 55
= ( 5 + 15 + 25 + 35 + 45 + 55 ) - ( 10 + 20 + 30 + 40 + 50 )
= ( 5 + 55 + 15 + 45 + 25 + 35 ) - ( 10 + 50 + 20 + 40 + 30 )
= ( 60 + 60 + 60 ) - ( 60 + 60 + 30 )
= 180 - 150
= 30
Vậy 5 - 10 + 15 - 20 + 25 - 30 + 35 - 40 + 45 - 50 + 55 = 30