What is the expected number of intermediate cells by age 45?
The probability that an intermediate cell will mutate to become a malignant cell is 5 × 10−7 per year.

Cancer
The two-stage model of carcinogenesis is based on the premise that for a cancer to develop, a normal cell must first undergo a “first hit” and mutate to become a susceptible or intermediate cell. An intermediate cell then must undergo a “second hit” and mutate to become a malignant cell. A cancer
develops if at least one cell becomes a malignant cell. This model has been applied to the development of breast cancer in females (Moolgavkar et al. [15]).
Suppose there are 108 normal breast cells and 0 intermediate or malignant breast cells among 20-year-old females. The probability that a normal breast cell will mutate to become an intermediate cell is 10−7 per year.