4 identical red balls and 5 identical white balls are arranged in a row, and the 4 red balls are not adjacent to each other. How many ways are there?
将4个同样的红球和5个同样的白球排成一排，要求4个红球互不相邻，共有多少种排法．
<gadget id="calculator">6 * 5 * 4 * 3</gadget>
<output>360</output>

<gadget id="calculator">4 * 3 * 2 * 1</gadget>
<output>24</output>

<gadget id="calculator">360 / 24</gadget>
<output>15</output>

<result>15</result>