Let x be the age of Mark, and y be the age of Adam. From the relationships given, we can formulate the following equations:
Eqn 1: x + y = 28
Eqn 2: x = 20 + y
Subsituting Eqn 2 to Eqn 1,
20 + y + y = 28
2y = 28 - 20 = 8
y = 8/2 = 4
Therefore, Adam is 4 years old.