There are 20 pieces of white leather in the soccer ball, which is 4 pieces less than twice the amount of black leather. How many pieces of black leather are there?
足球的白色皮共有20块，比黑色皮的2倍少4块．共有多少块黑色皮？
<gadget id="calculator">20 + 4</gadget>
<output>24</output>

<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<result>12</result>