Answer
$t = 118~min$
Work Step by Step
Note that the mass of 1.00 liter of water is 1.00 kg
We can find the energy required to melt half of the water:
$Q = L_v~m$
$Q = (2256~kJ/kg)(1.00~kg)$
$Q = 2256~kJ$
We can find the required time:
$t = \frac{E}{P}$
$t = \frac{2256~kJ}{(0.80)(400~W)}$
$t = 7050~s$
$t = 118~min$
