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x-8-{7-[6-(5-2)]}=7
1+2+3+...+18+19+20-x=5


Răspuns :

X-8-{7-[6-(5-2)]}=7
X-8-[7-(6-3)]=7
X-8-(7-3)=7
X-8-4=7
X=7+12
X=19

1+2+3+...18+19+20-x=5
(20x21):2-X=5
420:2-X=5
210-X=5
X=210-5
X=205