In a subtraction formula, the sum of the minuend, the subtrahend and the difference is 120, the difference and the subtrahend are equal, what is the difference?
在一个减法算式里，被减数、减数、差三数之和为120，差和减数相等，差是多少？
<gadget id="calculator">120 / 2</gadget>
<output>60</output>

<gadget id="calculator">60 / 2</gadget>
<output>30</output>

<result>30</result>