it depends what you want to invesigate.

You have observed that the coin landed on tails less than half the time. An obvious investigation would be to see if this could be due to random chance (ie, the real probability is 50% and you just got unlucky), or if it suggests that the real probability of landing tails-up is less than 50%.

So you would do a hypothesis test to see if your results are significantly different from a 50/50 split.

The hypothesis test could be based on the binomial distribution but with 100 samples you would want to use a normal approximation, unless you are allowed to use a computer.

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