Given: For cuboid shape box, breadth (b) = 6 unit, height (h) = 5 unit Total surface area = 500 sq. unit.
To find: Length of the box (l)
Total surface area of the box = 2(lb + bh + lh)
∴ 500 = 2 (6l + 6 x 5 + 5l)
∴ 500/2 = (11l + 30)
∴ 250 = 11l + 30
∴ 250 – 30= 11l
∴ 220 = 11l
∴ 220 = l
∴ 220/11 = l
∴ l = 20 units
∴ The length of the box is 20 units.