In algebra, the operation of substitution can be applied in various contexts involving formal objects containing symbols; the operation consists of systematically replacing occurrences of some symbol by a given value. Substitution is a basic operation of computer algebra. The method of solving "by substitution" works by solving one of the equations (you choose which one) for one of the variables (you choose which one), and then plugging this back into the other equation, "substituting" for the chosen variable and solving for the other. An example of substitution: 'I bet you get married [A] before I get married [A]. repetition. 'I bet you get married [A] before I do [B].