The store shipped 390 kg of oranges and apples. The mass ratio of oranges to apples is 6:7. How many kilograms are oranges less than apples?
商店运来橘子和苹果一共390千克．橘子和苹果的质量比是6：7．橘子比苹果少多少千克？
<gadget id="calculator">6 + 7</gadget>
<output>13</output>

<gadget id="calculator">7 / 13</gadget>
<output>7/13 = around 0.538462</output>

<gadget id="calculator">390 * (7/13)</gadget>
<output>210</output>

<gadget id="calculator">6 / 13</gadget>
<output>6/13 = around 0.461538</output>

<gadget id="calculator">390 * (6/13)</gadget>
<output>180</output>

<gadget id="calculator">210 - 180</gadget>
<output>30</output>

<result>30</result>