You have some unknown integer 
, and you know that adding this and the next two integers, 
and 
, gives a total of 57.
This means
The task is to find all three unknown integers. Notice that if you know the value of, then you pretty much know the value of the other three integers.
To find, solve the equation above:
So if 18 is the first integer, then others must be 19 and 20.
This means
The task is to find all three unknown integers. Notice that if you know the value of
To find
So if 18 is the first integer, then others must be 19 and 20.