In the ratio formula 4:15=8:30, if the latter term of the first ratio is increased by 5, how should the former term of the second ratio be changed to make this ratio valid?
在比例式4：15=8：30中，如果第一个比的后项增加5，那么第二个比的前项应该怎样变化才能使此比例成立？
<gadget id="calculator">4 * 30</gadget>
<output>120</output>

<gadget id="calculator">15 + 5</gadget>
<output>20</output>

<gadget id="calculator">120 / 20</gadget>
<output>6</output>

<gadget id="calculator">8 - 6</gadget>
<output>2</output>

<result>2</result>