Type A sugar is 8 yuan per kilogram, and type B sugar is 7.5 yuan per kilogram. After mixing 4 kilograms of type B candy with several kilograms of type A candy, the average mixed sugar is 7.8 yuan per kilogram. How many kilograms of type A sugar are used?
甲种糖每千克8元，乙种糖每千克7.5元，用4千克乙种糖果和若干千克甲种糖果混合后，平均每千克混合糖是7.8元，问甲种糖用了多少千克？
<gadget id="calculator">4 * 7.8</gadget>
<output>31.2</output>

<gadget id="calculator">7.5 * 4</gadget>
<output>30</output>

<gadget id="calculator">31.2 - 30</gadget>
<output>1.2</output>

<gadget id="calculator">8 - 7.8</gadget>
<output>0.2</output>

<gadget id="calculator">1.2 / 0.2</gadget>
<output>6</output>

<result>6</result>