After subtracting how many 30s in a row from 390, there are 30 left.
从390里连续减去多少个30以后，还剩30．
<gadget id="calculator">390 / 30</gadget>
<output>13</output>

<gadget id="calculator">13 - 1</gadget>
<output>12</output>

<result>12</result>