The average of three numbers is 120, plus one number, the average of four numbers is 115, what is this number?
三个数的平均数是120，加上一个数，四个数的平均数是115，这个数是多少．
<gadget id="calculator">115 * 4</gadget>
<output>460</output>

<gadget id="calculator">120 * 3</gadget>
<output>360</output>

<gadget id="calculator">460 - 360</gadget>
<output>100</output>

<result>100</result>