Respostas

2014-03-25T10:58:34-03:00
20-[-10+20-(-20+10)] = 20-[-10+20-(-10)]= 20-(-10+20+10)= 20-20 = 0 
2014-03-25T11:12:09-03:00
20-{-10+20-(-20+10)]}=
20-{-10+20-(-10)]}=
20-{-10+20+10}=
20-{20}=
20-20=0