The ratio of M and N is 4:5, if the latter term of the ratio is increased by 15, how much should the former term of the ratio be increased to keep the ratio constant.
M、N两数的比是4：5，如果比的后项增加15，要使比的比值不变，比的前项应该增加多少．
<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<gadget id="calculator">16 - 4</gadget>
<output>12</output>

<result>12</result>