Teacher Hua corrects homework continuously, and every time 20 homeworks are corrected, the correction speed will drop by 10% due to the decrease of energy. It takes 171 minutes for Teacher Hua to correct 40 homeworks. How many minutes does it take for Teacher Hua to correct 60 homeworks?
华老师连续批改作业，每批改20份作业由于精力降低批改速度会下降10%，华老师批改40份作业需要171分钟，那么华老师批改60份作业需要多少分钟．
<gadget id="calculator">10 / 9</gadget>
<output>10/9 = around 1.111111</output>

<gadget id="calculator">1 + (10/9)</gadget>
<output>19/9 = around 2.111111</output>

<gadget id="calculator">171 / (19/9)</gadget>
<output>81</output>

<gadget id="calculator">171 - 81</gadget>
<output>90</output>

<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 / (9/10)</gadget>
<output>10/9 = around 1.111111</output>

<gadget id="calculator">90 * (10/9)</gadget>
<output>100</output>

<gadget id="calculator">171 + 100</gadget>
<output>271</output>

<result>271</result>