Add 2.5 to the former term of 4:15, in order to keep the obtained ratio unchanged, how much should be added to the latter term of the ratio.
把4：15的前项加上2.5，为了要使所得的比值不变，比的后项应加上多少．
<gadget id="calculator">4 + 2.5</gadget>
<output>6.5</output>

<gadget id="calculator">6.5 / 4</gadget>
<output>1.625</output>

<gadget id="calculator">1.625 * 15</gadget>
<output>24.375</output>

<gadget id="calculator">24.375 - 15</gadget>
<output>9.375</output>

<result>9.375</result>