After the height of a cuboid is reduced by 3cm, it becomes a cube, and its surface area is reduced by 84cm square?

After the height of a cuboid is reduced by 3cm, it becomes a cube, and its surface area is reduced by 84cm square?

The cuboid can be seen as a combination of a cube and a cuboid 3 cm high (with a square at the bottom)
84 / 4 = 21
21 / 3 = 7 (this is the side length)
7 * 7 = 49 (bottom area)
3 + 7 = 10 (height of cuboid)
7 * 10 = 70 (area of a long aspect)
10 * 2 + 70 * 4 = 300 (result)
That's very detailed

If the height of a cuboid is increased by 3cm, it will become a cube, and the surface area will be increased by 84cm. What is the original volume?

84 ﹣ 4 ﹣ 3 = 7 cm
7-3 = 4cm
Original volume: 7 × 7 × 4 = 196 cubic centimeter

For a cuboid with a height of 40 cm and a square cross section, if the height is increased by 5 cm, the surface area and volume of the original cuboid will be calculated by adding 80 square cm

When the height is increased by 5cm, the increase of surface area is length * width * 5,
So, L * w * 5 = 80, L * w = 80 / 5 = 16 cm
Because the cross section is square, length = width = 4cm
The surface area of the original cuboid = (L * w * 2 + L * h * 2 + W * h * 2) = (4 * 4 * 2 + 4 * 40 * 2 + 4 * 40 * 2) = 672 square centimeters
Volume = L * w * H = 4 * 4 * 40 = 640 CC