After a bank's business hall opened, customers came to handle business one after another. If only one window is opened, the lobby will be full of customers in 15 minutes; if two windows are opened, the lobby will be full of customers in 30 minutes. Assuming that the number of customers who come to do business per minute remains the same, and the time it takes to handle the business of each customer is the same, how many windows should the bank open at least in order to ensure that customers can be handled at any time?
某银行营业厅开始营业后，顾客陆续前来办理业务．若只开一个窗口，15分钟后大厅中就站满顾客；若开两个窗口，30分钟顾客也能站满大厅．假设每分钟前来办事的顾客数不变，办理每个顾客的业务所用时间都相同，为了保障顾客随到随办理，此银行最少应开几个窗口？
<gadget id="calculator">2 * 30</gadget>
<output>60</output>

<gadget id="calculator">1 * 15</gadget>
<output>15</output>

<gadget id="calculator">60 - 15</gadget>
<output>45</output>

<gadget id="calculator">30 - 15</gadget>
<output>15</output>

<gadget id="calculator">45 / 15 / 1</gadget>
<output>3</output>

<result>3</result>