For a barrel of oil, 25.2 kilograms are used for the first time, and 1/4 of the total is used for the second time, and the result is 4.2 kilograms left. How many kilograms did the barrel of oil originally weigh?
一桶油，第一次用去25.2千克，第二次用去总数的(1/4)，结果还剩下4.2千克，这桶油原来重多少千克？
<gadget id="calculator">25.2 + 4.2</gadget>
<output>29.4</output>

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">29.4 / (3/4)</gadget>
<output>39.2</output>

<result>39.2</result>