Gọi số đó là a ta có:
\(\frac{12+a}{23-a}=\frac{2}{5}\)
\(\Rightarrow\left(12+a\right).5=\left(23-a\right).2\)
\(\Rightarrow60+5a=46-2a\)
\(\Rightarrow46-\left(60+5a\right)=2a\)
\(\Rightarrow46-60-5a=2a\)\(\)
\(\Rightarrow46-60=5a+2a\)
\(\Rightarrow7a=-14\)
\(\Rightarrow a=-2\)