The magnetic length of a bar magnet is 10cm then it's geometric length is

Geometric length: Geometric length is the measure of total length of magnet. A bar magnet consist of two equal and opposite magnetic pole separated by a small distance. Poles are not exactly at the ends. The shortest distance between two poles is called effective length and is less than its geometric length.

Magnetic length = (5/6) geometric lengthTherefore geometric length = 12 cm