This problem belongs to conditional probability problem.

Suppose event A: Animals live from birth to 20 years old, P(A)=0.8.

Event B: Animals live from birth to 25 years old, P(B)=0.4.

Then the probability that a 20-year-old animal will live to be 25 years old is P(A|B).

therefore

P(A | B)= P(AB)/P(A)= P(B)/P(A)= 0.4/0.8 = 0.5

A: The probability that an animal will live from 20 to 25 is 0.5.