There are two barrels containing a total of 50 kg of oil. If 20% is poured out of the first barrel and 4 kg is poured into the second barrel, the oil in the two barrels is equal. What is the original amount of oil in the first barrel?
有两只桶共装油50千克，若从第一桶里倒出20%，第二桶倒进4千克，则两桶油内的油相等，第一桶原来装油多少千克．
<gadget id="calculator">50 + 4</gadget>
<output>54</output>

<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">1 + (4/5)</gadget>
<output>9/5 = around 1.8</output>

<gadget id="calculator">54 / (9/5)</gadget>
<output>30</output>

<result>30</result>