hi this sound like a tricky question for me.
How many sequences of 2n coin flips have exactly n heads and n tails. my first impression is that the answer should be (2n choose n) * (2n choose n).
but the question is similar to say How many sequences of 2n coin flips have exactly n heads? because for 2n size, if you flip the coin 2n times and get n heads, it will always result in n tails right? so the correct answer should be (2n choose n) only is that right?