bucket of water weighs 23.5 kg, and a bucket of liquid medicine weighs 13.5 kg. The weight of the two empty barrels is the same, and the weight of the water is equivalent to twice the weight of the liquid medicine. What is the weight of an empty barrel in kilograms?
一桶水重23.5千克，一桶药液重13.5千克．两个空桶的重量一样重，并且水的重量相当于药液重量的2倍．求一个空桶的重量是多少千克？
<gadget id="calculator">13.5 * 2</gadget>
<output>27</output>

<gadget id="calculator">27 - 23.5</gadget>
<output>3.5</output>

<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">3.5 / 1</gadget>
<output>3.5</output>

<result>3.5</result>