As a cold front moved through a city, the temperature dropped rapidly from $\displaystyle{{78}}^{\circ}\text{F}\usepackage{cmbright}\Huge\$ , falling $\displaystyle\frac{{2}}{{5}}\usepackage{cmbright}\Huge\$ of a degree each minute. The change in temperature t minutes after the cold front moved through could be represented by the expression $\displaystyle-\frac{{2}}{{5}}{t}+{78}\usepackage{cmbright}\Huge\$ . What was the temperature 20 minutes after the cold front moved through?