There are four possible sequences of flips. Each is equally likely.
- Heads, Heads
- Heads, Tails
- Tails, Heads
- Tails, Tails
Since since the combination of tails twice in a row is one of four, you can calculate the probability as
This can also be done by multiplying the probability of a particular outcome by the probability for each trial. You know that the probability of tails for each trial is