The ratio of A and B is 4:5. If the latter term of the ratio is increased by 20, how much must the former term of the ratio be increased to keep the ratio constant.
甲、乙两数的比是4：5，如果比的后项增加20，那么比的前项必须增加多少才能使比值不变．
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<result>16</result>