A coin is flipped twice. What is the probability that it will come up once heads and once tails

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