Answer
$t = 34.9~min$
Work Step by Step
Note that the mass of 2.00 liters of water is 2.00 kg
We can find the energy required to raise the temperature of the water:
$Q = c~m~\Delta T$
$Q = (4187~J/kg~K)(2.00~kg)(80~K)$
$Q = 669,920~J$
We can find the required time:
$t = \frac{E}{P}$
$t = \frac{669,920~J}{(0.80)(400~W)}$
$t = 2093.5~s$
$t = 34.9~min$
