After 20 minutes the temperature is $\displaystyle-\frac{{2}}{{5}}{\left({20}\right)}+{78}\usepackage{cmbright}\Huge\$ . You added the change in temperature to the original temperature. But the temperature actually went down. That is, you calculated $\displaystyle-\frac{{2}}{{5}}{\left({20}\right)}\usepackage{cmbright}\Huge\$ and got a positive number. But a negative number times a positive number is negative.