Which expression represents 81 decreased by an unknown number?

A. 81-a

B. 81a

C. 81:a

D. 81 + a

Answer: The answer is A. 81 - 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.

Answer: Choice A

Step-by-step explanation:

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.