The average of three numbers is 13.5, the sum of the last two numbers is 27.5, what is the first number.
三个数的平均数是13.5，其中后两个数的和是27.5，第一个数是多少．
<gadget id="calculator">13.5 * 3</gadget>
<output>40.5</output>

<gadget id="calculator">40.5 - 27.5</gadget>
<output>13</output>

<result>13</result>