3aa1 is divisible by 9, ask how much a=. (Question 3 of the third test of the Long Island Mathematics Competition in the United States)
3aa1能被9整除，问a=多少．(美国长岛数学竞赛第三试第3题)
<gadget id="calculator">18 - 4</gadget>
<output>14</output>

<gadget id="calculator">14 / 2</gadget>
<output>7</output>

<result>7</result>