Answer
It is according to the characteristics of a binomial random variable, page 196.
Work Step by Step
1. We have a sample of 100 lab mice cells, each one representing a trial.
2. Success: a cell responds positively. Failure: a cell responds negatively.
3. $p=.4$ and $q=1-p=.6$
4. The response of each cell is independent of the response of the other cells
5. $x$ is the number of times a cell responds positively.
