Input the number of random arrivals ( such as births ) and the time (in days ) during which the observations were recorded.
You will get a probability distribution of the number of arrivals you can expect per day.
Then, enter a number of concern, and get the probability that you you observe fewer than or equal to that number.

This is a study involving the Poisson Distribution.
Do some research online to find the formula for it.
What is the mean for this distribution?
What is the standard deviation?