It means the number of possible sequences of heads and tails would be:
2¹² = 4096
We can determine the number of ways that such a sequence could contain exactly 9 tails is the number of ways of choosing 9 out of 12, using the formula
Plug in n = 12 and r = 9
∵
∵
Thus, the probability will be:
Thus, the probability of the coin landing tails up exactly nine times will be: