The milk production of 3 goats and 3 cows in 3 days is equal to the milk production of 5 goats and one cow in 4 days. If a goat produces 5 liters of milk a day, how many liters of milk does a cow produce in a day?
3只山羊和3头奶牛3天的产奶量与5只山羊一头奶牛4天的产奶量相等，如果一只山羊一天产奶5升，那么一头奶牛一天产奶多少升．
<gadget id="calculator">5 * 4</gadget>
<output>20</output>

<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">20 - 9</gadget>
<output>11</output>

<gadget id="calculator">5 * 11</gadget>
<output>55</output>

<gadget id="calculator">1 * 4</gadget>
<output>4</output>

<gadget id="calculator">9 - 4</gadget>
<output>5</output>

<gadget id="calculator">55 / 5</gadget>
<output>11</output>

<result>11</result>