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21-4*[1+15/(a-9)]=15

Răspuns :

21-4×[1+15 :(a-9)]=15/+21
4×[1+15 ÷(a-9)]=36/÷4
1+15÷(a-9)=9/+1
15÷(a-9)=10/×15
a-9=150
a=150+9
a=159