Question

write the number of ways that a given system of forces acting on a rigid can be replaced by two forces equivalent to the given system.​

Answer 1

A concurrent coplanar force system is a system of two or more forces whose lines of action ALL intersect at a common point. However, all of the individual vectors might not acutally be in contact with the common point. These are the most simple force systems to resolve with any one of many graphical or algebraic options.
force system with all of the arrows drawn parallel
A parallel coplanar force system consists of two or more forces whose lines of action are ALL parallel. This is commonly the situation when simple beams are analyzed under gravity loads. These can be solved graphically, but are combined most easily using algebraic methods.
a force system in which none of the arrows meet at one single point and they are not parallel. Essentially a jumble of forces.