The average of the three numbers A, B, and C is 7.5, and the half of the sum of the numbers B and C is 4.5. How much is armor?
甲、乙、丙三个数的平均数是7.5，乙、丙两数和的一半是4.5．甲是多少？
<gadget id="calculator">7.5 * 3</gadget>
<output>22.5</output>

<gadget id="calculator">4.5 * 2</gadget>
<output>9</output>

<gadget id="calculator">22.5 - 9</gadget>
<output>13.5</output>

<result>13.5</result>