The circuit model of a dc voltage source is shown in $($Figure $1).$ The following voltage measurements are made at the terminals of the source: $(1)$ With the terminals of the source open, the voltage is measured at $80$ mV $,$ and $(2)$ with a $\backslash (15\backslash$mathrm$\{$M$\}\backslash$Omega $\backslash )$ resistor connected to the terminals, the voltage is measured at $73$ mV $.$ All measurements are made with a digital voltmeter that has a meter resistance of $\backslash (12\backslash$mathrm$\{$M$\}\backslash$Omega $\backslash ).$

Figure

Part A

What is the internal voltage of the source $\backslash (\backslash$left$($v$\_\{$s$\}\backslash$right$)\backslash )$ in millivolts?

Express your answer in millivolts to three significant figures.