There are numerous important information's already given in the question. Based on these information's, the answer can be deduced.
Let us assume the age of Daniel = x years
Let us assume the age of Kevin = y years
Then
y = 4x
and
y = x + 6
So
4x = x + 6
4x - x = 6
3x = 6
x = 6/3
= 2 years
So
Age of Kevin = 4 * 2 years
= 8 years
