Answer
50.6 volts is the emf, and 1.2 ohms is the internal resistance.
Work Step by Step
Equation 19-1 relates the emf, the current, and the terminal voltage.
$$V=\epsilon-Ir$$
We are given information about the terminal voltage and the current in 2 situations.
$$V_1=\epsilon-I_1r$$
$$V_2=\epsilon-I_2r$$
Solve these 2 equations simultaneously for the two unknowns.
$$\epsilon=V_1+I_1r=V_2+I_2r$$
$$r=\frac{V_2-V_1}{I_1-I_2}=\frac{47.3V-40.8V}{8.40A-2.80A}\approx 1.2\Omega$$
$$\epsilon=V_1+I_1r=40.8V+(8.40A)(1.161\Omega)=50.6V$$