The correct answer is choice A. The equation 81 - a is doing exactly what the question is asking for. It represents 81 minus an unknown number, with the unknown number being shown as a.
Step-by-step explanation: decreased is practically a synonym for subtract, so we use the symbol for subtraction (-). 81 decreased by an unknown number, which then we use a symbol to show the unknown variable, like a letter. So (a) represents the unknown number. Hence, 81 decreased by an unknown number becomes 81 - a.
