A box is of 10 cm long, 8 cm broad and 5 cm high. What is

A box is of 10 cm long, 8 cm broad and 5 cm high. What is the longest possible length of a pencil that can be put in ?

Since, the box is in the form of a cuboid.
∴ Required length = √10² +8² + 5² cm
= √189
= √9 x 21
= 3√21 cm