Hello,

From your text, we can say that , where R represents the event "egg rotten" and A the even "egg provided by A".

We also have

What we are looking for is , that is to say "if the egg is rotten(condition), what is the probability that it was supplied by A (event)"

Now, let X be the number of eggs supplied by B.

A will supply 3X eggs.

The total is 4X.

So the probability to take an egg from A is

With the formula of conditional probability, we have

Hence