Let x be the original amount of money Jimmy had. Since he spent 12 and has the fifth of x this we have:
[tex]\begin{gathered} x-12=\frac{x}{5} \\ x-\frac{x}{5}=12 \\ \frac{4}{5}x=12 \\ x=\frac{12}{\frac{4}{5}} \\ x=15 \end{gathered}[/tex]Then, he had $15 originally.
