notez cu a numărul.
75÷[(a×2-20)÷30+13]=5
(a×2-20)÷30+13=75÷5
(a×2-20)÷30+13=15
(a×2-20)÷30=15-13
(a×2-20)÷30=2
a×2-20=2×30
a×2-20=60
a×2=60+20
a×2=80
a=80÷2
a=40
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Verificare: 75÷[(40×2-20)÷30+13]=5
75÷[(80-20)÷30+13]=5
75÷(60÷30+13)=5
75÷(2+13)=5
75÷15=5