We have a cuboid whose length is 4 unit and breadth is 8 unit and hight is 10 unit. To find surface area of cuboid first we find the area of all faces of cuboid then sum of area of all faces is equal to surface area of cuboid.

Cuboid has 6 faces two opposite faces of dimension is 4 and 8 other two opposite faces of dimension is 8 and 10 and last two opposite faces is 10 and 4.
Rectangular face of dimension 4 and 8 has area = 4 x 8 and similar surface on opposite side whose area is also 4 x 8.
Rectangular face of dimension 8 and 10 has area = 8 x 10 and similar surface on opposite side whose area is also 8 x 10.
Rectangular face of dimension 10 and 4 has area = 10 x 4 and similar surface on opposite side whose area is also 10 x 4.

Total surface area = axb +axb +bxc +bxc +cxa +cxa
= 2ab +2bc +2ca
= 2(ab + bc + ca)
= 2(4 x 8 + 8 x 10 + 10 x 4) = 2x 152 = 304 unit area.