Buy a barrel of oil, pay 37.1 yuan. This barrel of oil weighs 8 kilograms, and the weight of the barrel after half of the oil is used is 4.5 kilograms. How much is it per kilogram of oil?
买一桶油，付37.1元．这桶油连桶重8千克，用去一半油后连桶重4.5千克．每千克油多少元？
<gadget id="calculator">8 - 4.5</gadget>
<output>3.5</output>

<gadget id="calculator">3.5 * 2</gadget>
<output>7</output>

<gadget id="calculator">37.1 / 7</gadget>
<output>5.3</output>

<result>5.3</result>