Answer
Please see the work below.
Work Step by Step
We know that
$H=\frac{\Delta }{R}$
We also know that
$Q=Ht$
and $Q=mL_f$
$\implies Ht=mL_f$
This can be rearranged as:
$m=\frac{\Delta Tt}{RL_f}$
We plug in the known values to obtain:
$m=\frac{(20)(15\times 60\times 60)}{(0.12)(334000)}$
$m=27Kg$
