For a barrel of oil, half of the oil is poured out each time. After pouring it three times, the barrel weighs 8 kilograms. It is known that the barrel weighs 3 kilograms. How many kilograms of oil are there in the barrel?
一桶油，每次倒掉油的一半，倒了三次后连桶重8千克，已知桶重3千克，原来桶里有油多少千克？
<gadget id="calculator">8 - 3</gadget>
<output>5</output>

<gadget id="calculator">5 * 2 * 2 * 2</gadget>
<output>40</output>

<result>40</result>