Answer:
There is a 81.86% probability that the obstetrician has delivered no child with polydactyly.
Step-by-step explanation:
There are only two possible outcomes: Either the baby has the anormality, or he hasn't. So we use the binomial probability distribution.
Binomial probability
The binomial probability is the probability of exactly x successes on n repeated trials, and X can only have two outcomes.

In which
is the number of different combinatios of x objects from a set of n elements, given by the following formula.

And
is the probability of X happening.
In this problem, we have that:
It is reported in about one child in every 500, so
.
A young obstetrician celebrates her first 100 deliveries, so 
What is the probability that the obstetrician has delivered no child with polydactyly?
That is P(X = 0)


There is a 81.86% probability that the obstetrician has delivered no child with polydactyly.