Systems of measurement sorry you pagal this

Geometric unit systems are useful in relativistic physics. In these systems the base physical units are chosen so that the speed of light and the gravitational constant are set equal ...

Planck units are a form of geometric units obtained by also setting the reduced Planck constant to unity. ...

Stoney units are a form of geometric units obtained by also setting the Coulomb constant and the elementary charge to unity

Explanation:

If you want the meaning also here

A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement in use include the International System of Units, the modern form of the metric system, the British imperial system, and the United States customary system.

81 is a composite number.