Gọi số đó là \(\overline{ab}\left(a,b\inℕ\right)\). Theo đề ta có :
\(\overline{a1b}-7\cdot\overline{ab}=4\)
\(a\cdot100+10+b-7\left(a\cdot10+b\right)=4\)
\(a\cdot100+10+b-a\cdot70-b\cdot7=4\)
\(a\cdot30+10-b\cdot6=4\)
\(a\cdot15+5-b\cdot3=2\)
\(3\left(a\cdot5+b\right)+5=2\)
\(3\left(a\cdot5+b\right)=2-5\)
\(3\left(a\cdot5+b\right)=-3\)
\(a\cdot5+b=-\frac{3}{3}=-1\left(!!!\right)\)
=> sai đề