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5+[32-(10:a×4+12)]×7=89

Răspuns :

[32-(10:ax4+12)]x7=84 (89-5)
32-(10:ax4+12)=12 (84:7)
10:ax4+12=20 (32-12)
10:ax4=8 (20-12)
10:a=2 (8:4)
a=10:2
a=5