\(\Rightarrow100a+10b+3=\frac{3}{4}\left(300+10a+b\right)\)
\(\Rightarrow100a+10b+3=225+7,5a+0,75b\)
\(\Rightarrow\left(100a-7,5a\right)+\left(10b-0,75b\right)=225-3\)
\(\Rightarrow92,5a+9,25b=222\)
\(\Rightarrow9,25.10a+9,25b=222\)
\(\Rightarrow9,25\left(a0+b\right)=222\)
\(\Rightarrow ab=222:9,25\)
\(\Rightarrow ab=24\)