The stadium (Bird's Nest), the main venue of the Beijing 2008 Olympic Games, has an actual investment of 3.1 billion yuan, which is (1/5) less than the original plan. How many billion yuan was the original plan?
北京2008年奥运会主会场体育馆(鸟巢)，实际投资31亿元，比原计划节约了(1/5)．原计划造价多少亿元？
<gadget id="calculator">1 / 5</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>