The average of three numbers is known to be 13, and the average of two of them is 15, what is the third number
已知三个数的平均数是13，其中两个数的平均数是15，第三个数是多少
<gadget id="calculator">13 * 3</gadget>
<output>39</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">39 - 30</gadget>
<output>9</output>

<result>9</result>