Răspuns :
10 + ( -20 ) = -10
20 + ( - 30 ) = -10
30 + ( - 40 ) = -10
40 + ( - 50 ) = - 10
50 + ( - 60 ) = - 10
20 + ( - 30 ) = -10
30 + ( - 40 ) = -10
40 + ( - 50 ) = - 10
50 + ( - 60 ) = - 10