Quantity I: The ratio of the ages of A and B is 3:1, and the ratio of the ages of B to C is 3:5. If C’s age after 20 years equals A’s current age, and D’s age is 80% more than B’s age, what is the average age of A and D? Quantity II: A is 12 years older than B, and B is 10 years younger than
C. The average age of A, B, C, and D is 30 years. If C’s age equals D’s age, find D’s age after 4 years.