First sugar is 5.60 yuan per kilogram, and second sugar is 9.8 yuan per kilogram. After mixing 4 kilograms of first sugar and how many kilograms of second sugar, the average price is 7 yuan per kilogram.
甲糖每千克5.60元，乙糖每千克9.8元，4千克甲糖与多少千克乙糖混合后，平均每千克售价7元．
<gadget id="calculator">7 * 4</gadget>
<output>28</output>

<gadget id="calculator">5.6 * 4</gadget>
<output>22.4</output>

<gadget id="calculator">28 - 22.4</gadget>
<output>5.6</output>

<gadget id="calculator">9.8 - 7</gadget>
<output>2.8</output>

<gadget id="calculator">5.6 / 2.8</gadget>
<output>2</output>

<result>2</result>