A batch of 81kg of white sugar was shipped in. In the first 15 days, it sold 3.6kg a day, and then it sold an average of 4.5kg a day. How many days will it take to sell it all?
运来一批白糖81kg，前15天，每天卖3.6kg，以后平均每天卖4.5kg，再用几天可以卖完？
<gadget id="calculator">3.6 * 15</gadget>
<output>54</output>

<gadget id="calculator">81 - 54</gadget>
<output>27</output>

<gadget id="calculator">27 / 4.5</gadget>
<output>6</output>

<result>6</result>