A Bernoulli random variable describing whether two molecules are together (both in or both out) or separate

Question:

A Bernoulli random variable describing whether two molecules are together (both in or both out) or separate at time l.
Think about one or more molecules independently leaving a cell, each with probability 0.9 in a given second. Find the random variable describing the following events, find the probabilities of the outcomes, and find the expectation.