Final answer:
The probability of heads on the fourth trial is 1/2 or 0.5.
Step-by-step explanation:
The probability of flipping a fair coin and landing on heads is always 50%, regardless of previous tosses. Each coin toss is an independent event, meaning that the outcome of one toss does not affect the outcome of another. So, if a penny is tossed three times and comes up heads all three times, the probability of heads on the fourth trial is st