A barrel of oil is poured out all (1/4) for the first time, and all (2/5) is poured out for the second time, leaving 7 kilograms. How many kilograms is this barrel of oil originally? (solution by equation)
一桶油第一次倒出全部的(1/4)，第二次倒出全部的(2/5)，还剩7千克，这桶油原来有多少千克？(用方程解)
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (1/4) - (2/5)</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">7 / (7/20)</gadget>
<output>20</output>

<result>20</result>