A barrel of oil weighs 21 kilograms, and the weight of the barrel after (1/3) of the oil is used up is 15 kilograms. How many kilograms does the barrel weigh?
一桶油，连桶重21千克，用去(1/3)的油后连桶重15千克，桶重多少千克？
<gadget id="calculator">21 - 15</gadget>
<output>6</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">6 / (1/3)</gadget>
<output>18</output>

<gadget id="calculator">21 - 18</gadget>
<output>3</output>

<result>3</result>