Fahim is as much younger than Bilash as he is older than Wasim. If the sum of the ages of Wasim and Bilash is 66 years, then what is definitely the difference between Fahim’s and Wasim's age?

Let,
Fahim's age = x
Bilash's age = y and
Wasim's age = z.

According to the given information
Age of Bilash - Age of Fahim = Age of Fahim - Age of Wasim
⇒ y - x = x - z
⇒ 2x = y + z ......(i)
Also, y + z = 66
from (i) x = 33 years

but, y + z = 66 means, there can be many sets of values that will satisfy the equation.
01 - 65
02 - 64
... ... ...
32 - 34

We want to determine x - z but as we can't get a fixed value for z; hence, the answer can not be determined.