It is known that A*B=12, B*C=14, C*D=21. So how much is A*D=?
已知A*B=12，B*C=14，C*D=21．那么A*D=多少．
<gadget id="calculator">12 * 21</gadget>
<output>252</output>

<gadget id="calculator">252 / 14</gadget>
<output>18</output>

<result>18</result>