There are three colors of red, white, and black balls in a pile. The total number of white balls and red balls is 16. There are 7 more red balls than black balls, and 5 more black balls than white balls. How many black balls are there?
一堆球中有红、白、黑三种颜色，白球和红球合起来是16个，红球比黑球多7个，黑球比白球多5个，那么黑球有多少个．
<gadget id="calculator">16 - 7</gadget>
<output>9</output>

<gadget id="calculator">5 + 9</gadget>
<output>14</output>

<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">14 / 2</gadget>
<output>7</output>

<result>7</result>