After the two barrels of oil in A and B are each used up to 12 kilograms, the oil in barrel A is exactly three times the oil in barrel B. It is known that barrel A originally had 36 kilograms of oil, how many kilograms of oil did barrel B have?
甲乙两桶油各用去12千克后，甲桶油就正好是乙桶油的3倍，已知甲桶原有36千克油，乙桶原有多少千克油？
<gadget id="calculator">12 * 3</gadget>
<output>36</output>

<gadget id="calculator">36 - 12</gadget>
<output>24</output>

<gadget id="calculator">36 + 24</gadget>
<output>60</output>

<gadget id="calculator">60 / 3</gadget>
<output>20</output>

<result>20</result>