The printing factory binds a batch of textbooks, 36,000 volumes are bound every day, and it takes 15 days to complete. If 4,000 more volumes are bound every day, how many days can it be completed in advance? (Solution with proportional knowledge)
印刷厂装订一批课本，每天装订3.6万册，需要15天完成．如果每天多装订0.4万册，可以提前几天完成？(用比例知识解)
<gadget id="calculator">3.6 * 15</gadget>
<output>54</output>

<gadget id="calculator">0.4 * 15</gadget>
<output>6</output>

<gadget id="calculator">6 - 54</gadget>
<output>-48</output>

<gadget id="calculator">54 + (-48)</gadget>
<output>6</output>

<gadget id="calculator">3.6 + 0.4</gadget>
<output>4</output>

<gadget id="calculator">6 / 4</gadget>
<output>3/2 = around 1.5</output>

<result>3/2 = around 1.5</result>