It boils down to recognizing:
Doing some practise papers for my DM final and have come across something I don't quite understand. I didn't do high school math (last time I did any was 8 years ago) so I find this course very challenging/frustrating but enjoyable!
But the actual/final answer should be:
Can anyone explain how to get this? I understand factorials, but only the basics when variables are involved. Thankyou!