A barrel of gasoline is used up (4/7), and 4 kilograms are added after arriving at the gas station. At this time, the gasoline in the barrel is exactly half. How many kilograms is this barrel of gasoline originally?
一桶汽油用去(4/7)，到加油站后加进4千克，这时桶里的汽油正好有一半．这桶汽油原有多少千克？
<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(4/7) - (1/2)</gadget>
<output>1/14 = around 0.071429</output>

<gadget id="calculator">4 / (1/14)</gadget>
<output>56</output>

<result>56</result>