[tex]15\ is\ 30\%\ of\ x\\\\30\%=\frac{30}{100}=\frac{3}{10}=0.3\\\\15\ is\ 0.3\ of\ x\\\\therefore\\\\0.3x=15\Rightarrow x=15:0.3\\\\\boxed{x=50}\\\\other\ method:[/tex]
[tex]\begin{array}{cccc}30\%&-&15&|divide\ both\ sides\ by\ 3\\\\10\%&-&5&|multiply\ both\ sides\ by\ 10\\\\100\%&-&50&-solution\end{array}[/tex]