After 2/5 of the total mass of a barrel of oil is used up, another 120 kg is bought. At this time, the mass of the oil is exactly 90% of the original. How many kg of oil are there? (column equation solution)
一桶油，用去总质量的(2/5)后，又买来120千克，这时油的质量恰好是原来的90%，原来有油多少千克？(列方程解答)
<gadget id="calculator">0.9 - 0.6</gadget>
<output>0.3</output>

<gadget id="calculator">120 / 0.3</gadget>
<output>400</output>

<result>400</result>