In the ratio 7:10=21:30, if the second item increases its (2/5), then how much must the fourth item increase to establish the ratio.
比例7：10=21：30中，如果第二项增加它的(2/5)，那么第四项必须增加多少，比例才能成立．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">10 * (2/5)</gadget>
<output>4</output>

<gadget id="calculator">10 + 4</gadget>
<output>14</output>

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

<gadget id="calculator">14 * 3</gadget>
<output>42</output>

<gadget id="calculator">42 - 30</gadget>
<output>12</output>

<result>12</result>