[tex] let \: \\ x = the \: number \: of \: pennies \: the \\ \: bank \: holds\\ 120 + \frac{1}{2}x = \frac{2}{3}x \\ 120 = \frac{2}{3}x - \frac{1}{2}x \\ 120 = \frac{4}{6}x - \frac{3}{6}x \\ \frac{1}{6}x = 120 \: so \\ \: x = 120 \times 6 = 720[/tex]
