A barrel of oil is used (2/5) on the first day, and 5 kilograms are used on the second day, and half is left. How many kilograms of this barrel of oil are there?
一桶油，第一天用去(2/5)，第二天用去5千克，还剩一半，这桶油原来有多少千克？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(1/2) - (2/5)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">5 / (1/10)</gadget>
<output>50</output>

<result>50</result>