Efficiency = \(\frac{power output}{power input}\) = \(\frac{R}{R + r}\) Given efficiency = 75% = 0.75, R = 6.0\Omega, r = ? Efficiency = \(\frac{R}{R + r}\) 0.75 = \(\frac{6.0}{6.0 + r}\) 0.75(6.0 + r) = 6.0 4.5 + 0.75r = 6.0 0.75r = 6.0 - 4.5 = 1.5 r = \(\frac{1.5}{0.7}\) = 2\Omega The internal resistance r of the cell is 2.0 ohms.