There is a pile of Go pieces, the number of white pieces is twice the number of black pieces, after taking away 6 white pieces, the number of black pieces is twice the number of white pieces, how many black pieces are there?
有一堆围棋子，白子数是黑子数的2倍，拿走6个白子后，黑子数是白子数的2倍，黑子有多少个．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 - (1/2)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">6 / (3/2)</gadget>
<output>4</output>

<result>4</result>