Finding the areas of each of the rectangles and squares of the net of a rectangular prism and adding up those areas gives the surface area or total surface area of the prism. For example, if the length of one side of the cube 4 units then the area of one its face is 4 × 4 = 16 square units.
If V = IR, then V = 40 * 3 in this case, which means V = 120. Quote question, "Assume voltage remains constant." This means V still equals 120 in the second scenario, which we can input into I = V / R to make I = 120 / 20 = 6.