When subtracting two numbers, if the minuend is increased by 6, the difference should be increased by 15, what is the number of the subtrahend?
两个数相减，如果被减数增加6，要使差增加15，减数应该多少．
<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<result>9</result>