The actual investment of the National Stadium, 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 actual cost saved? (solution by equation)
北京奥运会主会场国家体育馆实际投资31亿元，比原计划节约了20%，实际造价比原来节约了多少亿元？(用方程解)
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">31 * (1/5)</gadget>
<output>31/5 = around 6.2</output>

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

<gadget id="calculator">(31/5) / (4/5)</gadget>
<output>31/4 = around 7.75</output>

<result>31/4 = around 7.75</result>