In an ancient country, 1 cow can be exchanged for 5 pigs, 1 pig can be exchanged for 3 sheep, and 4 cows can be exchanged for how many sheep.
古代一个国家，1头牛可以换5头猪，1头猪可以换3头羊，4头牛可以换多少头羊．
<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<result>60</result>