After 70% of a barrel of oil is used up, 10 kilograms of gasoline is poured into the barrel. At this time, the gasoline is exactly half of the original gasoline. How many kilograms of gasoline are there?
一桶油，用去70%后，又向桶内倒入10千克汽油，这时汽油正好是原来汽油的一半．原来汽油有多少千克？
<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>