A certain factory burns a pile of coal, initially planning to burn 3.9 tons per day, and burn it out in 15 days. Actually burned for 20 days, what percentage is actually saved every day?
某工厂烧一堆煤，最初打算每天烧3.9吨，15天烧完．实际烧了20天，实际每天节省百分之几？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) - (1/20)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/60) / (1/15)</gadget>
<output>1/4 = around 0.25</output>

<result>1/4 = around 0.25</result>