VERY URGENT!!!! problem from Module 0, week 2 challenge, question 15

Can somebody explain why it needs to multiply 15 with a 2?
I have put the question and the whole explanation below. Thank you so much~

@victorioussheep Because every pair of circles is touching at 2 points. In order to solve this problem, you need to find the number of pairs of circles, which can be done by using the choose equation. After you get 15 pairs, you have to multiply it by 2 to get the total number of intersections because each pair is intersecting at 2 points.

@coherentmango thank you so much!