The average of 4 numbers is 7.3, the last three numbers are 6.1, 7.2, 9.5, what is the first number.
4个数的平均数是7.3，后三个数是6.1，7.2，9.5，第一个数是多少．
<gadget id="calculator">7.3 * 4</gadget>
<output>29.2</output>

<gadget id="calculator">6.1 + 7.2 + 9.5</gadget>
<output>22.8</output>

<gadget id="calculator">29.2 - 22.8</gadget>
<output>6.4</output>

<result>6.4</result>