Uncle Zhang in the cafeteria bought 0.5 kilograms of beans at the vegetable market, handed it to the salesperson for 10 yuan, and got back 7.9 yuan. How much per kilogram of beans?
食堂张叔叔在菜场买0.5千克豆角，交给售货员10元钱后，找回7.9元．每千克豆角多少钱？
<gadget id="calculator">10 - 7.9</gadget>
<output>2.1</output>

<gadget id="calculator">2.1 / 0.5</gadget>
<output>4.2</output>

<result>4.2</result>