Total surface area = 2(lw + lh + wh) = 2(100+20+20) = 280 m². Choose the correct formula first, substitute the given values carefully, and match the final unit with the type of measurement. A cuboid has three different pairs of equal rectangular faces: lw, lh, and wh. Add these three face areas and multiply by 2 to include the opposite faces, then use square units.