The actual investment in the main venue of the Beijing Olympic Games is 3.1 billion yuan, which is 20% less than the original plan. How much is the original plan to invest?
北京奥运会主会场馆实际投资31亿元，比原计划节约了20%，原计划投资多少亿元？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">31 / (4/5)</gadget>
<output>155/4 = around 38.75</output>

<result>155/4 = around 38.75</result>