In a subtraction formula, the sum of the minuend, the subtrahend and the difference is equal to 120, and the subtrahend is 3 times the difference, so what is the difference.
在一个减法算式里，被减数、减数与差的和等于120，而减数是差的3倍，那么差等于多少．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">120 / 2 / 4</gadget>
<output>15</output>

<result>15</result>