15 years later, A will be twice as old as B but five years ago A was 4 times as old as B. Find the difference of their present ages ?

30 years

Let A's age = x years and B's age = y years
As per the first condition,
∴ (x + 15) = 2(y + 15)
⇒ x - 2 y = 15 ....(i)

As the per second condition,
∴ (x - 5) = 4(y - 5)
⇒ x - 4 y = -15 ....(ii)

Solving (i) and (ii) one get's, x = 45, y = 15
∴ A's age = 45 years
B's age = 15 years

∴ Difference of their ages = 45 - 15 = 30 years