One barrel of oil is used up (4/5), and another 15 kilograms is added. At this time, the gasoline in the barrel is exactly (1/2) of the whole barrel of gasoline. How many kilograms of this barrel of gasoline are there?
一桶油用去(4/5)，又加进15千克，这时桶里的汽油正好是全桶汽油的(1/2)，这桶汽油原有多少千克？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 - (4/5)</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/2) - (1/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">15 / (3/10)</gadget>
<output>50</output>

<result>50</result>