There are two opaque boxes, black and white, each box contains a number of black balls and white balls, if the white ball is taken out of the black box, 1 point is added, and if the black ball is taken out of the white box, 2 points are added, otherwise no Add points. If Xiaogang has a score of 15 after taking 10 balls from the two boxes, how many black balls can Xiaogang take out from the two boxes at most?
有黑白两个不透明的箱子，每个箱中都装有若干黑球和白球，若从黑箱中取出白球，则加1分，若从白箱中取出黑球，则加2分，其他情况不加分．如果小刚从两个箱中取了10次球后的得分是15，那么小刚从两个箱中取出的黑球最多有多少个？
<gadget id="calculator">7 * 2</gadget>
<output>14</output>

<gadget id="calculator">15 - 14</gadget>
<output>1</output>

<gadget id="calculator">10 - 7 - 1</gadget>
<output>2</output>

<gadget id="calculator">7 + 2</gadget>
<output>9</output>

<result>9</result>