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daca  21- 8 ₊ 7 - x = 31, atunci numarul x este egal cu...

Răspuns :

21-8+7-x=31 13+7-x=31 20-x=31 X=31-20 X=11
21-8+7-x=31 8+7-x=31-21 8+7-x=10 7-x=10+8 7-x=18 x=18-7 x=11 S={11}