https://h.vn/hoi-dap/question/62675.html
\(\frac{72-x}{7}=\frac{x-40}{9}\)
\(\Leftrightarrow\left(72-x\right).9=7.\left(x-40\right)\)
\(\Leftrightarrow648-9x=7x-280\)
\(\Leftrightarrow648-280=7x+9x\)
\(\Leftrightarrow368=16x\)
\(x=368:16\)
\(x=23\)
vậy x=23
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