Pack a batch of apples. If 42 boxes have been packed, the remaining apples are 70% of this batch of apples; if 85 boxes are packed, there are still 1540 apples left. How many apples are there in this batch?
一批苹果装箱．如果已装了42箱，剩下的苹果是这批苹果的70%；如果装了85箱，则还剩下1540个苹果．这批苹果共有多少个？
<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 - (7/10)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">42 / (3/10)</gadget>
<output>140</output>

<gadget id="calculator">140 - 85</gadget>
<output>55</output>

<gadget id="calculator">1_540 / 55</gadget>
<output>28</output>

<gadget id="calculator">28 * 140</gadget>
<output>3_920</output>

<result>3_920</result>