Four cubes, each of edge 5 cm are joined end to end. What is the total surface area of the resulting cuboid ?

When four cubes are joined end to end,

Length of the resulting cuboid $$(l)$$ = 4 x 5 = 20 cm

Breadth of the resulting cuboid $$(b)$$ = 5 cm

Height of the resulting cuboid $$(h)$$ = 5 cm

Total Surface Area of the cuboid = $$2\left[lb+bh+lh\right]=2\left[\left(20\right)\left(5\right)+\left(5\right)\left(5\right)+\left(20\right)\left(5\right)\right]=2\left[100+25+100\right]=2\left(225\right)=450  cm^2$$