In the answer given, the mu_(70+t) is derived from q_70. But since mu_70 is directly available in the Tables, why cannot we directly use mu_70 for this calculation?
When we're told to assume that the force of mortality is constant over a certain year of age, we set it equal to the average force of mortality over that year. Mu(70) from the Tables, is the force of mortality at exact age 70, not the average force between exact age 70 and exact age 71. The average force is equal to the integral from 0 to 1 of mu(70+t) dt.
