tails
heads
Yes.
Tails !! I win !!
20
tails
It was heads
tails
It was heads.
heads
heads
The probability is 0%. The result will be heads or it will be tails but it cannot be heads and tails.
Yes.
It is 100%. The coin will result in heads or tails since there are no other possible outcomes.
7
Tails !! I win !!
20