In a subtraction formula, the sum of the minuend, the subtrahend, and the difference between these three numbers is 104, and the difference is three times the subtrahend. What is the difference?
在一道减法算式里，被减数，减数，差这三个数的和是104，差是减数的3倍，差是多少．
<gadget id="calculator">1 + 3 + 1 + 3</gadget>
<output>8</output>

<gadget id="calculator">104 / 8</gadget>
<output>13</output>

<gadget id="calculator">3 * 13</gadget>
<output>39</output>

<result>39</result>