In a box of black and white chess pieces, the number of black stones is the total number (3/5). After putting 12 white stones into the box, the number of black stones accounts for the total number (3/7). How many black stones are there in the box?
一盒黑、白棋，黑子的颗数是总数的(3/5)，把12颗白子放入盒子后，黑子的颗数占总数的(3/7)，盒子里原有黑子多少颗．
<gadget id="calculator">7 - 3</gadget>
<output>4</output>

<gadget id="calculator">4 / 3</gadget>
<output>4/3 = around 1.333333</output>

<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">(4/3) - (2/3)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">12 / (2/3)</gadget>
<output>18</output>

<result>18</result>