find the value of 'a-b' if " a "and "b" are integers such that b is the predecessor of "a"

Answer :

We know " The predecessor of a integer is one less than the given number.Clearly, the predecessor of 1 is 0; predecessor of 2 is 1; predecessor of 3 is 2 and so on. " 
And
integers = { ... , -3 , -2 , -1 , 0 , 1, 2 , 3, ... } 

So,
if we have b as a predecessor of a than value of  ( a - b ) = 1              ( always )
let a  = 1 than b = 0 ( as b is predecessor of a ) 
So,
a - b = 1 - 0 = 1
Or 
Let a = - 2 ,So b  =  - 3 ​( as b is predecessor of a ) 
a - b = - 2 + 3 = 1 

So,
a  - b  = 1 , when b is predecessor of a .                    ( Ans)