Expand the former term of 8:13 to 4 times of the original, how much should the latter term be increased to keep the ratio constant.
将8：13的前项扩大到原来的4倍，要使比值不变，后项应增加多少．
<gadget id="calculator">52 - 13</gadget>
<output>39</output>

<result>39</result>