The length of the edges of the cuboid is 20 cm. The length of the longest stick that can be placed in it is equal to the distance from one corner to the corner on the opposite face that lies diagonally opposite to it.
This length is equal to L = `sqrt(20^2 + 20^2 + 20^2)` using the Pythagorean Theorem.
L = `sqrt(3*400)`
=> `sqrt 1200`
=> L = 34.64
The length of the longest stick that can be placed in a cuboid with edges 20 cm is 34.64 cm