A school originally had 30 basketballs and volleyballs, of which 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 were bought later? (solution by equation)
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%，后来买来了几个排球？(用方程解)
<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">30 * (2/5)</gadget>
<output>12</output>

<gadget id="calculator">7 + 3</gadget>
<output>10</output>

<gadget id="calculator">3 / 10</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">30 * (3/10)</gadget>
<output>9</output>

<gadget id="calculator">12 - 9</gadget>
<output>3</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">3 / (3/5)</gadget>
<output>5</output>

<result>5</result>