Your answer would be 25%.
Assuming the coin is not biased, the probability of flipping a heads or tails is 1/2 equally. This remains the same no matter how many times you flip the coin, because the outcome of one flip does not affect the outcome of another.
This means that the probability of getting a tails the first flip is 1/2, and the probability of then getting a heads is also 1/2, so to find the probability of both happening you need to multiply these probabilities together, and 1/2 × 1/2 = 1/4, and 1/4 as a percentage is 25%.
I hope this helps! Let me know if you have any questions :)