Hi to all of you :)

Even though I am decent at coding usually math heavy questions in tests get me. For instance this question:

Have the function BracketCombinations(num) read num which will be an integer greater than or equal to zero, and return the number of valid combinations that can be formed with num pairs of parentheses. For example, if the input is 3, then the possible combinations of 3 pairs of parenthesis, namely: ()()(), are ()()(), ()(()), (())(), ((())), and (()()). There are 5 total combinations when the input is 3, so your program should return 5.

The answer is:

int(factorial(2 * num) / (factorial(num   1) * factorial(num)))

I can understand the factorial(2 * num) part but rest is mystery. Can you explain it? or better direct me to resources that I can learn these concepts in depth with lots of examples, sample questions etc. It really bothers me if the part I am stuck is math in a coding test.