4.x≠-3;
2(x+3)=30 =>x+3=15=>x=15-3=>x=12" alt="\frac{2}{5}= \frac{6}{x+3}=>2(x+3)=30 =>x+3=15=>x=15-3=>x=12" align="absmiddle" class="latex-formula">
6. x≠0 ;
\frac{12}{\frac{4x}{5} }=\frac{20}{\frac{1}{4} }=>\frac{15}{x}=80=>x=\frac{3}{16}" alt="12:\frac{4x}{5} = 20:\frac{1}{4}=> \frac{12}{\frac{4x}{5} }=\frac{20}{\frac{1}{4} }=>\frac{15}{x}=80=>x=\frac{3}{16}" align="absmiddle" class="latex-formula">