Question

what are the main system of units​

Answer 1

A system of units is a set of related units that are used for calculations. ... For example, in the MKS system, the base units are the meter, kilogram, and second, which represent the base dimensions of length, mass, and time, respectively. In this system, the unit of speed is the meter per second.

Answer 2

It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units.