After half of a barrel of oil is used up, another 30 kilograms is poured in. At this time, the weight of the oil in the barrel is the original (4/5). How many kilograms of oil are there in the barrel?
一桶油用去一半后，又倒进30千克，这时桶内油的重量是原来的(4/5)，桶内原来有油多少千克？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(4/5) - (1/2)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">30 / (3/10)</gadget>
<output>100</output>

<result>100</result>