You probably correctly subtracted the first bill to get $\displaystyle{230}-{127.63}={102.37}\usepackage{cmbright}\Huge\$ , but then incorrectly subtracted the second bill. $\displaystyle{102.37}-{108.02}={102.37}+{\left(-{108.02}\right)}\usepackage{cmbright}\Huge\$ will have a negative result. Tracy will then be overdrawn and another $35 should be subtracted.