Let's call the unknown amount of money x
Betty spend 5/12 of her money (5/12)x and had 42 dollars left. Translated,
[tex]x - \frac 5{12} x = 42[/tex]
Solving,
[tex]x(1 - \frac 5{12}) = 42[/tex]
[tex] \frac{7}{12} x = 42[/tex]
[tex] x = \dfrac{12}{7} \cdot 42 = 72[/tex]
Answer: $72
