A is 3 years older to B and 3 years younger to C, while B and D are twins. How many years older is C to D?

Explanation:-

Since B and D are twins, so B = D.
Now, A = B + 3 and A = C - 3.
Thus, B + 3 = C - 3 ⇔ D + 3 = C-3 ⇔ C - D = 6.