I fail to understand what a certain symbol means, please explain.
It is in my permutation and combination chapter and it is for arrangements of 7 digits where something repeats thrice and something repeats twice. So the answer is correct that I understand.
But what is this symbol doing? Please help
The symbol https://i.stack.imgur.com/oQHTu.jpg and the symbol in context https://i.stack.imgur.com/uKDGY.jpg
