There are some black and white pieces in the box. After adding 10 white pieces, the white pieces account for black and white pieces (2/5). After adding 20 black pieces, the white pieces account for black and white pieces (1/3). How many white pieces are there? ?
盒子里有一些黑子和白子，加入10颗白子后，白子占黑、白子的(2/5)，再加入20颗黑子后，白子占黑、白子的(1/3)，原来有白子多少颗？
<gadget id="calculator">5 - 2</gadget>
<output>3</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">2 - (3/2)</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">20 / (1/2)</gadget>
<output>40</output>

<gadget id="calculator">40 - 10</gadget>
<output>30</output>

<result>30</result>