Xiaoming's family has 28 rabbits of three kinds. There are 20 gray rabbits and white rabbits, and 12 gray rabbits and black rabbits. How many white rabbits are there?
小明家养了三种兔子共28只，灰兔和白兔一共有20只，灰兔和黑兔一共有12只，白兔有多少只．
<gadget id="calculator">12 - 28</gadget>
<output>-16</output>

<gadget id="calculator">20 + (-16)</gadget>
<output>4</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<result>16</result>