Answer
$\Delta t=456s=7.60\ min$
Work Step by Step
With $W_e=\Delta H\rightarrow\dot{W}_e\Delta t=m\Delta h_v$:
Given $\Delta t=25\ min, m=0.5\ kg,\ \Delta h_v=2256.4\ kJ/kg$
we get: $\dot{W}_e=0.752\ kW$
For $\dot{W}_e\Delta t=mc\Delta T$
Given $m=1\ kg,\ \Delta T=(100-18)K, c_v=4.18\ kJ/kg.K$:
$\Delta t=456s=7.60\ min$
