A is 50, B is 30% of A, how much is B.
甲是50，乙是甲的30%，乙是多少．
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">50 * (3/10)</gadget>
<output>15</output>

<result>15</result>