Step-by-step explanation:
so, "plan" means the view straight from the top. the 3 cubes seen are in the front.
"front elevation" means the view straight from the front.
"side elevation" means the view straight from a side (e.g. right). let's say the left column is the front side (from the side).
that means, as seen via "plan", we have originally a full front face (12 cubes).
then the 2nd face (from right to left) has 2-0-0 cubes.
the 3rd face has also 2-0-0 cubes.
and the 4th face has 1-1-0 cubes.
for the completely filled up cuboid we need 12 cubes (3×4) in each face.
so, we added 10 cubes in each of the 2nd, 3rd and 4th face.
that means we added 30 cubes.