school originally had 30 basketballs and volleyballs, and the ratio of basketballs to volleyballs was 7:3, and a few more volleyballs were bought in. At this time, the number of volleyballs accounted for 40% of the total, how many volleyballs should be bought.
某校原有篮球和排球共30个，其中篮球与排球的比是7：3，又买进几个排球，这时排球的个数占总数的40%，则买进多少个排球．
<gadget id="calculator">7 + 3</gadget>
<output>10</output>

<gadget id="calculator">30 / 10</gadget>
<output>3</output>

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

<gadget id="calculator">100 - 40</gadget>
<output>60</output>

<gadget id="calculator">40 / 60</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">(2/3) - (3/7)</gadget>
<output>5/21 = around 0.238095</output>

<gadget id="calculator">21 * (5/21)</gadget>
<output>5</output>

<result>5</result>