Using deductive reasoning, you can create a system of equations to represent the two numbers: $\displaystyle{a}+{b}={8}\usepackage{cmbright}\Huge\$ , and $\displaystyle{2}{a}={b}+{1}\usepackage{cmbright}\Huge\$ . Isolating one of the variables and then substituting it into the other equation is a form of deductive reasoning, as you are taking some known information (the equations) and changing them—according to mathematical rules!—to create new information. Solving the system, you find that $\displaystyle{a}={3}\usepackage{cmbright}\Huge\$ and $\displaystyle{b}={5}\usepackage{cmbright}\Huge\$ .