It is known that a:b=c:d, now a is enlarged by 2 times, b is reduced by 2 times, c remains unchanged, and the ratio of d is still valid.
已知a：b=c：d，现在将a扩大2倍，b缩小2倍，c不变，d多少，比例依然成立．
<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<result>4</result>