After 70% of a barrel of oil is used up, 10 kg of gasoline is poured into the barrel. At this time, the amount of oil in the barrel is exactly half of a full barrel of oil, how many kilograms is a full barrel of oil.
一桶油在用掉70%之后，又向桶内倒入10千克汽油．这时桶内的油量刚好是一整桶油的一半，一整桶油有多少千克．
<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">1 - (7/10)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">(1/2) - (3/10)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">10 / (1/5)</gadget>
<output>50</output>

<result>50</result>