a, 10 - ( 5 + 10 ) + 30
= 10 - 5 - 10 + 30
= ( 10 - 10 ) + ( 30 - 5 )
= 0 + 25
= 25
b, ( - 5 - 15 ) + ( - 5 - 20 )
= -5 - 15 - 5 - 20
= - ( 5 + 5 ) - ( 20 + 15 )
= - 10 - 35
= - ( 10 + 35 )
= -45
c, 70 - ( 120 - 70 - 100 )
= 70 - 120 + 70 + 100
= ( 70 + 70 ) - 120 + 100
= 140 - 120 + 100
= 20 + 100
= 120