There are ballpoint pens of various colors in the box, among which the red ballpoint pens account for (1/4). Later, 8 red ballpoint pens are put into the box. At this time, the red ballpoint pens account for (5/12) of the total number of red ballpoint pens.
盒里装着各色圆珠笔，其中红色的占(1/4)后来又往盒里放了8支红色圆珠笔，这时红色圆珠笔占总数的(5/12)则原有红色圆珠笔多少支．
<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">(5/7) - (1/3)</gadget>
<output>8/21 = around 0.380952</output>

<gadget id="calculator">8 / (8/21)</gadget>
<output>21</output>

<gadget id="calculator">21 * (1/3)</gadget>
<output>7</output>

<result>7</result>