There are 6 points on a circle, and any two points are connected by a line segment, how many points of intersection do these line segments have at most in the circle?
圆上有6个点，任意两点用线段相连，则这些线段在圆内最多有多少个交点．
<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>