Answer
The mass of the mud is $1.9\times10^5kg$.
Work Step by Step
We calculate that the volume of that section of mud to be:
$(2500m)(800m)(2m)=4\times10^6m^3$
We are then told this mud is distributed on a $400m\times400m$ surface. Since the total volume won't change, we can calculate the new depth.
$(2500m)(800m)(2m)=(400m)(400m)x$
$x=25m$
The volume over an area of $4m^2$ can be calculated by:
$V=(4m^2)25m=100m^3$
Each cubic meter has a density of $1900kg/m^3$. To calculate the mass of that portion of mud we:
$m=(1900kg/m^3)(100m^3)=1.9\times10^5kg$
