Intermediate Algebra
Because variables are represented by letters and coefficients or constants are numbers, the rules of arithmetic forbid replacing numbers with other numbers, so only letters (variables) are substituted.
We substitute variables because coefficients and constants are always integers, and replacing the integers would change the units in the expression.
Variables represent unknown or varying quantities, so we replace them with values. Coefficients and constants are fixed numbers that define the expression and therefore are not replaced.
Variables must be substituted first so that coefficients and constants can later be substituted during simplification.