It is about force of mortality:

In a population initially consisting of 75% females and 25% males, you are given:

(i) For a female, the force of mortality is constant and equals u .

(ii) For a male, the force of mortality is constant and equals 1.5u .

(iii) At the end of 20 years, the population consists of 85% females and 15%males.

Calculate the probability that a female survives one year.