In the ratio 7:10=21:30, if the second item increases by 80%, then the fourth item must be increased by how much, the ratio can be established.
在比例7：10=21：30中，如果第二项增加它的80%，那么第四项必须增加它的多少，比例才能成立．
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">10 * (4/5)</gadget>
<output>8</output>

<gadget id="calculator">10 + 8</gadget>
<output>18</output>

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

<gadget id="calculator">18 * 3</gadget>
<output>54</output>

<gadget id="calculator">54 - 30</gadget>
<output>24</output>

<result>24</result>