Hazel is three times as old as Joel, and in 4 years she will be twice as old as Joel. How old was Hazel when Joel was born?

If,

J = Joel's age now, then

H = 3J is the Hazel's age

After 4 years,

Hazel's age = H+4 = 3J+4

Joel's age = J+4

But,

Hazel's age = 2*Joel's age

Then,

3J+4 = 2 (J+4)

3J+4 = 2J+8

J = 8-4 = 4 years

And,

Hazel's age now = J+4 = 4+4 = 8 years

Therefore, Hazel was 4 (8-4) years when Joel was bone.