Answer
$f=27.95\%$
Work Step by Step
From the energy balance:
$\dot{Q}+\dot{m}h_1=\dot{m}h_2$
$q=c_p\Delta T+\Delta h_v$
$q_h=c_p\Delta T$
$f=\frac{q_h}q =\dfrac{c_p \Delta T}{c_p\Delta T+\Delta h_v}$
Given $c_p=4.18\ kJ/kg,\ \Delta T=180°C,\ \Delta h_v=1939.8\ kJ/kg$
$f=0.2795=27.95\%$
