1/2 or 50%, because there are two possibilities out of four.
If you think about flipping a coin twice, there are exactly four possible outcomes:
Flip 1 Flip 2
1. tails heads
2. tails tails
3. heads tails
4. heads heads
In two of these possibilities (1 and 3) the coin flipped different sides.
That means 2/4 possibilities matched, and this can be reduced to 1/2
